I have a "homemade" dual exhaust with the stock cats and stock GT mufflers. this has made a huge difference and cost me a whole 30 for the mufflers(used) and a sat of cutting and welding. you would probly get the same effect with a 2.5" exhaust with no cats and a NICE flowmaseter or magnaflow or whatever you prefer. Eventualy im gona take off the cats and throw glass packs on my car...oh and im running 2.5" pipe.