I just got the GSX and have been looking at Downpipes, catbacks, test pipes, and 02 housings. My delemma is I'm not sure what size I should go with. I was think of doing 3" on everything but then I heard if you go that big on the 02 sensor and downpipe you loose room in your engine bay. I have also been told that a 2.5 inch downpipe and 02 sensor are the best cause the exhaust gases are hotter there and don't expand until it reaches the catback area. I am also looking into different catbacks. I want one with a dual tipe like stock but can't find that. Should I get a 3" or 2.5"? is the 3" that much better? Thanks guys!

2.5" o2 housing and downpipe, (2.5 expanding to 3" DP would be best), 3" test pipe, 3" catback. If you get dual tips it just hurts flow. Go with the normal 3" single tip.

I believe its www.roadraceengineering.com that sells the 3" inlet into dual tip.

Wait on the exhaust housing intil you picked your turbo you might go with. Otherwise you might end up replacing it again and losing money