This is a question where you will get many different opinions. Although you can get too big of an exhaust that actually hampers the scavenging of exhaust gasses a 2.5" or 3" IMO would be OK. I would probably go with the 2.5" depending on what your future plans include. If they include boost or higher lift cams, some other type of power adder then I would probably go with the 3". You could definately do better in the header department though. I have the DC w/ 2.5" collector and it is one of the main downfalls of my build. As soon as I have some $$$ it will be an RMF b16 header that will far outflow the DC. I read an article where they had an ITR with a straight 3" exhaust, which was basically a long test pipe and it made way more power than the aftermarket 2.5" exhaust they had on. I'm bored and have a keyboard so take what you want out of this but I would say, go to Mainstream Performance and get Charles to build you a custom 2.5" exhaust. You will come out much better and have something that is truely hand made and custom.