I did this on a eaton set up.... custom 2.5 x pipe and Maga flo cat back .... went to a jba shorty flo mater off road x pipe and jba 3 c cat back .... at a 520 hp I lost power it seemed to bog and not as torquey ....same set up but with a whipple....580 @17 and 710 @24 on e85...pulls hard on the top but not real crisp down low .... I think 3" dose not have enough back pressure till u start hitting big boost or high hp
I would almost guess something else is responsible for your car not feeling real crisp down low. I have 1 7/8 long tubes, 3" o/r X and 3" cat back and can't tell the slightest bit of difference in how it drives down low. Honestly I'd need a dyno and swap between 2.5" X and catback and the 3" to see what the differences are and where each one is better.