you talking about a catback system? i'd go with a 2.5" piping for sure, 3" on a naturally aspiratted QR25 is too big. I feel 2.25 is too small too. 2.5 is your best option. i've done a bit of research and i have found that the BRM exhaust system gives the best power gains and is one of the most inexpensive systems out. it's your option of aluminized or stainless steel, 2.25-3" piping choices, you have the option of a resonator and it comes with i belive a free flowing non resonator muffler.. checkout
www.brmusa.com
Although, i'd get the header first cause the biggest gains are gonna come from that. your option on a header should be hotshot hands down. You also can get a downpipe, which are pretty cheap around 190 shipped from hpautoworks or megan racing.