umm dude, coilovers are suspension components. they dont make your car go, they just hold it off the ground. for the stage you are at, i rekon the exhaust is the next step. but dont just do a catback cos they do nothing but add noise. do the full exhaust. ie: catback, hiflow cat, front/dump pipe.
imo you are looking at maybe $300 for a secondhand catback, max $300 depending on which cat u chose, and bout $200 to $300 for the front pipe - bout $900 total. i spent almost that much on a new trust PE2 catback. stupid move but meh.
look for a bargain on the forums, they crop up regularly. you'll definately feel the difference