On a N/A motor I think you need that back pressure for your low end grunt. That's why you see some lose some trq when they swap out to a off road pipe. I know I did when I had my 2000 GT, and that was a H-pipe w/cats.
You will pick it back up on the top end though, just matters where you want it. I would stick with the the 2 1/2 set-up. Now get blown and that's another story.