Exhaust question's?
 

I want to get a budget 2.5" B-pipe from my local shop. I would like to try and keep the same muffler but I don't think it would make much of a difference if I swap the B-pipe to 2.5" but the muffler inlet is only 2.25". Is it worth it to swap the B-pipe leaving the factory muffler or should I just swap the muffler out with the B-pipe? I'm contemplating getting the Powertech Magnaflow muffler. Also should I get the B-pipe with or without resonator? How will this affect the noise output? I don't want the noise to increase much.
The guy also said that unless I plan on Supercharging it's not worth it to swap out exhaust. I'm kicking around the idea of getting a 50 or 75 shot Wet kit in the future. And my mods to this date are intake & UDP.

exhaust systems on NA cars don't add much hp.

Ammi is right. A catback won't add much hp...but when you combine that with a y-pipe you can get some pretty decent hp gains from that whole setup. b-pipe w/ resonator will be less noisy than a b-pipe w/o a resonator.

How much does the resonator affect the flow or gains?

if you haven't done any mods, I would go w/ Y Pipe instead of exhaust system. You'll definitely see some gain in power.