I gained 2 mph consistently with the MAC 3" over the 2.5 Stinger kit I had. I also noticed a considerable seat-of-the-pants difference top end. I did lose a little down low on torque on the dyno, but it pulls harder up top.
I do have plans for a twin screw though and that is the only reason I got the 3".
But for the record, before the 3" I ran 120, now I run 121.5-122 consistently.
I like the 3.5 tips the best, but my old Borla
4" tips looked sick. If you do go 2.5 though, buy the SLP double wall tips....those are awesome. I even have a brand new unused set if you are interested.