Jelly, 7 bytes

ȮL×Ċ”)x

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Uses the decimal format.

How?

ȮL×Ċ”)x - Main link: string, decimal
Ȯ       - print string
 L      - length(string)
  ×     - multiply by the decimal
   Ċ    - ceiling (since rounding method is flexible)
    ”)  - a ')' character
      x - repeated that many times
        - implicit print

Perl 6, 21 bytes

{$^a~")"x$^b*$a.comb}

JavaScript ES6, 38 31 30 bytes

s=>n=>s+')'.repeat(s.length*n)

f=s=>n=>s+')'.repeat(s.length*n)

console.log(f("hi!")(4.99))

Common Lisp, 59 52

Parentheses? I am in.

(lambda(s n)(format()"~a~v{)~}"s(*(length s)n)'(_)))

Details

(lambda(s n)                  ; two arguments (string and ratio)
  (format ()                  ; format as string
          "~a~v{)~}"          ; control string (see below)
          s                   ; first argument (string)
          (* (length s) n)    ; second argument (number of parens)
          '(_)))              ; a dummy list of one element

Format control string

• ~a : pretty print argument (here the given string)
• ~v{...~} : iteration block, limited to V iteration, where V is taken as an argument, namely the (* ...) expression. The iteration is supposed to iterate over a list, which is why there is a dummy non-empty list '(_) given as an argument (an empty list would not work).

Since no element in the list is consumed by the format, the loop is infinite but fortunately, it is also bounded by V, a.k.a. the number of parentheses to be printed.

Edit: thanks to Michael Vehrs for pointing out that there is no need to round the numerical argument (the question allows to truncate/round however we want, so the default behavior works here).

Python, 30 bytes

lambda s,r:s+')'*int(len(s)*r)

Uses the decimal input.

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Python 2, 29 bytes

lambda s,p:s+len(s)*p/100*')'

s in the string, p is the percentage (integer).

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05AB1E, 9 8 bytes

g*ï')×¹ì

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g*       # Length, multiplied by emphasis.
  ï')×   # Covnerted to an integer, push that many parenthesis.
      ¹ì # Prepend original string.

Works for both integer and decimal, arguments order: f(String, Double)

PowerShell, 33 bytes

$a,$b=$args;$a+')'*($b*$a.Length)

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Supports decimal format.

R, 62 46 bytes

Anonymous function that takes string s and decimal n, prints output to stdout.

function(s,n)cat(s,rep(")",n*nchar(s)),sep="")

Pyth, 8 bytes

*\)s*lpz

Online Test! Takes the excitement ratio first, then the string to be enthused about.

Explanation:

      pz  print out the enthused string
     l    ... and get its length
    *...Q multiply that by the ratio
   s      floor to get an integer, let's call this S
 \)       single-character string ")"
* ")" S   multiply that integer by the string, which gives a string of )s of length S.
          implicitly print that string of S )s.

Pyth, 9 bytes

*s*lpzE")

Takes two lines of input: string and ratio (decimal).

Try it on pyth.herokuapp.com

Explanation

A denotes a function's first argument, B its second argument.

*s*lpzE")
    pz     # print the input string
   lAA     # take the length of the printed string
      E    # read the next line of input (the emphasis ratio)
  *AAAB    # multiply the length by the ratio
 sAAAAA    # floor the result
*AAAAAA")  # repeat ")" n times
           # implicit print

TI-Basic, 33 bytes

Takes decimal input.

Prompt Str1,A
")
For(I,0,9
Ans+Ans
End
Str1+sub(Ans,1,AI

CJam, 9 bytes

l_,ld*')*

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Input string on the first line, emphasis ratio in range 0 to 5 on the second.

Explanation

l    e# Read input string.
_,   e# Duplicate, get length.
ld   e# Read emphasis ratio.
*    e# Multiply by length.
')*  e# Get that many parentheses.

Perl 5, 29 bytes

sub{($_=pop).')'x(y///c*pop)}

(Number is first arg, string is second.)

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MATL, 11 10 8 bytes

yn*:"41h

This solution uses the decimal form of the second input

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Explanation

        % Implicitly grab first input as a string
        % Implicitly grab the second input as a number
y       % Make a copy of the first input
n       % Compute the length of the string
*       % Multiply the decimal by the length to determine the # of )'s (N)
:       % Create the array [1...N]
"       % For each element in this array
  41    % Push 41 to the stack (ACSII for ")")
  h     % Horizontally concatenate this with the current string
        % Implicit end of for loop and display

sB~, 17 bytes

i\,N?\;')'*(N*l(\

Explained:

i\,N    input a string and a number
?\;     print the string
')'*    also print ) multiplied by...
(N*l(\  the number times the string length.

Parentheses are closed automatically

Here's the output of the compiler, if you're interested:

 INPUT  S$ ,N? S$ ;")"*(N* LEN(  S$ ))

This version of the compiler was written on 1/27/2017 at 11:12 pm, which might have been a few minutes after this question was posted. So here's a version which works on the oldest version of the compiler, written an hour earlier: iS$,N?S$;')'*(N*l(S$)) (22 bytes)

Rebol, 39 bytes

func[a b][append/dup a")"b * length? a]

SimpleTemplate, 92 bytes

Takes the string as the first parameter and the "ratio" as the second.
The ratio is between 0 and 5, with 2 decimal places.

{@echoargv.0}{@callstrlen intoL argv.0}{@set*Y argv.1,L}{@callstr_repeat intoO")",Y}{@echoO}

As you can see, it is non-optimal.
The 2 {echo} there could be reduced to 1.
Due to a bug in the compiler, this code can't be reduced much further.

Ungolfed:

{@echo argv.0}
{@call strlen into length argv.0}
{@set* ratio argv.1, length}
{@call str_repeat into parenthesis ")", ratio}
{@echo parenthesis}

If no bug existed, the code would look like this, 86 bytes:

{@callstrlen intoL argv.0}{@set*Y argv.1,L}{@callstr_repeat intoO")",Y}{@echoargv.0,O}

C# Interactive, 77 67 bytes

string r(string s,int p)=>s+new string(')',(int)(s.Length*p/100d));

C# interactive is sweet.

PostgreSQL, 102 bytes

create function q(text,int)returns text as $$select rpad($1,(100+$2)*length($1)/100,')')$$language sql

Details

Uses the integer input format.

This simply right-pads the input string with parens out to the target length.

create function q(text,int)
returns text as $$
    select rpad($1,             -- Pad the string input
        (100 + $2) *            -- to 100 + int input % ...
        length($1) / 100,       -- ...of the input string
        ')')                    -- with ) characters
$$ language sql

Called with

select q('codegolf', 125), q('codegolf', 75);
select q('noob team omg', 50), q('hi!', 499);

SmileBASIC, 29 bytes

INPUT S$,N?S$;")"*(LEN(S$)*N)

Gol><> (Golfish), 17 bytes

i:a=?v
R*Il~/Hr)`

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The top line reads characters (i) until it finds a newline (ASCII 10, a), then goes down (v).

Then we discard one character (the newline) with ~, push the length of the stack (l), read a float (I), multiply the two, and repeatedly (R) push the character ")" that many times. Finally, reverse the stack (r), output it and halt (H).

Bash + coreutils, 45 bytes

echo $1`seq -s\) $[${#1}*$2/100+1]|tr -cd \)`

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Integer input.

PHP, 50 bytes

<?=str_pad($s=$argv[1],strlen($s)*++$argv[2],")");

takes string and decimal number as command line arguments; cuts padding. Run with -r;

breakdown

<?=                     // print ...
str_pad(                    // pad
    $s=$argv[1],            // string=argument 1
    strlen($s)*++$argv[2],  // to string length*(1+argument 2) 
    ")"                     // using ")" as padding string
);

Groovy, 27 bytes

Straightforward solution

{s,r->s+')'*(s.length()*r)}

Test program :

def f = {s,r->s+')'*(s.length()*r)}

println f("hi!", 4.99)
println f("noob team omg", 0.5)

Mathematica, 24 bytes

#<>Table[")",#2Tr[1^#]]&

Unnamed function taking a list of characters and a decimal numbr as input and returning a string. Tr[1^#] is a sneaky golfy way to calculate the length of a list, so #2Tr[1^#] computes the required number of parentheses. Table[")",...] produces a list of that many right parentheses (all decimals automatically rounded down, which is nice). Finally, #<> concatenates the input string to the parentheses, flattening the list produced by Table as it goes.

Clojure, 40 bytes

Quite boring solution :

#(reduce str %(repeat(*(count %)%2)")"))

Just reduces str function on a list of closing parentheses with a string as initial parameter.

See it online : https://ideone.com/5jEgWS

Not-so-boring solution (64 bytes) :

#(.replace(str(nth(iterate list(symbol %))(*(count %)%2)))"(""")

Converts input string to a symbol (to get rid of quotes) and repeatedly applies function list on it generating infinite sequence like this: (a (a) ((a)) (((a))) ... ). Takes nth element converts it to string and replaces all opening parentheses with nothing.

See it online : https://ideone.com/C8JmaU

Ruby, 25 bytes

->(s,n){s+')'*(s.size*n)}

I'm using lambdas. The test program would be something like:

f=->(s,n){s+')'*(s.size*n)}
f.("codegolf", 1.5)        # => "codegolf))))))))))))"
f.("hi!", 4.99)            # => "hi!))))))))))))))"

GNU AWK, 59 bytes

{t=sprintf("%"n*length(s)"s","");gsub(/ /,")",t);print s t}

Accepts arguments on command line (using the decimal form) like so:

: | awk -v s="codegolf" -v n=0.75 -f russianify.awk

Yes, AWK requires something to be attached to stdin, even if that something is 0 bytes returned by the no-op shell builtin. You could also use echo | awk ..., or really anything that doesn't contain a newline.

Clojure, 68 bytes

An anonymous function that accepts decimal input.

(fn [s n] (print (str s (reduce str (repeat (* n (count s)) ")")))))

Literally the first Lisp program I've ever written! I'm already having fun.