My brother had 2.5" and went with 3" when he still had the 350 in his olds. It ran in the low 14's - high 13's most of the time. While the change didn't provide a huge power increase, the engine did seem to rev more freely, and it stopped blowing collector gaskets out where the tubing size transitioned from the 3" collector to the 2.5" exhaust pipe.

And yes 3" equals more noise vs 2.5" pipe. Depending on where you live the cops might take an interest in the sound making capacity of your ride.