a really nice catback will give you an ample amount of power. after i got my Apexi put on my gs-r, i immediately noticed a gain in low-end torque and mid-high range hp. Of course, my old exhaust was getting clogged and robbed of power, but after i put my 4-2-1 dc header on it, the engine just hums beautifully. I would recommend getting a cat-back first, a NICE one. Don't get fooled by shops who custom make them and put a crappy muffler in the back. Mufflers are just as important as the whole exhaust.