Ain't no way in hades I'd swap out a 3" system I already had for a 2.5" system. I'm a huge proponent of leaving the exhaust the same diameter as "whatever" the header collector is. I don't care what the argument is or who makes it. A restriction is a restriction and going to a smaller pipe RIGHT off the header is a big restriction. I know. Here come the bullcrap arguments about "increased velocity" out of the warped imaginations of people who've probably never even made a dyno pull. AJ is usually so fulla crap his eyes are brown, but post #6 got it right.