I think I can actually feel a difference or I'm just driving it harder cuz I like the way it sounds? I drive alot of miles so I will have to watch for changes in mileage (a possible increase?) and vacuum/boost readings on hills. If mileage goes up and boost under cruise drops that would confirm a power increase due to better breathing. I should mention that the P.O. had removed the rear muffler, replacing it with a straight pipe so if you have the two muffler system I would expect that it would be even more noticable.
This is pretty subjective but I did notice on the entrance ramp this morning that just driving what I thought was normal that I picked up another 500 rpm's, meaning I have to watch my tach a little closer!