ok, correct me if I'm wrong, but you cant do TRUE dual exhaust on a I4 since theres only one downpipe... but you can do a y-pipe which splits the exhaust from one pipe to two... this is what my cars got factory, but I'm pretty sure that will rob you of power... since your turbo I'd say your best bet is to do single exhaust 2.75 - 3 inch all the way back.