The key is having the 2.5" collector on the header and a 2.5" inlet/outlet catalytic or testpipe to get the most power. Although a true 2.5" exhaust will give the best flow, the differences over a 2.36" (60mm) which is the more common JDM sized exhausts, will be very small if any. I'd think a true 2.5" exhaust would only net substantial gains on a highly modified motor... hell ive seen some cars pull nearly 190whp on the stock cat-back exhaust before.