I have a friend that has a 2.5in exhaust on a Mustang with 670whp. I think 2.5in is just fine for a turbo 4cyl, thats what i got.

if you want to spend the extra money for the 3in go ahead but its not worth. I have a bigger turbo right now, but when i was boosting 16psi on my other turbo i had stock downpipe and cat and had no problems. Of course with 2.5in from turbo back with no cat the car revs more freely.

all in all 2.5in is fine, plus it sounds a little better than 3in. some 3in exhaust are very loud. just make sure you get a good straight through muffler