Truth is that on th S197 GT's the gains of an aftermarket exhaust system over the stock system are minimal at best. The stock system has mandrel bends and at 2.5 inches is actually a bit oversized already. It's not a liability until you start going to forced induction. Most people who go with aftermarket exhausts in these cars do it for the sound. Borla S types and Pypes Violators seems to be pretty popular, personally I think Bassani has some nice sounding stuff for the 05+ GT's.