2.5" is most efficient for twin turbo's in my power range. Remember, that I have a true dual exhaust. 3" would be ideal if I had ONE exhaust pipe. I did alot of research before ordering, and experts have proven that 3" piping can actually hamper performance whereas 2.5" is perfect for my goals. I would not need dual 3" exhaust until I got in the 800+ HP range. I'm only shooting for @ 500rwhp.