removing smog equipment like EGR and such does not increase hp, exhaust would add about 4-8 whp for full exhaust, I put 2-1/2" exhaust w/ no cat, magnaflow muffler and headers, with intake, felt a little bit of difference, car ran smoother but thats about it.
Loud exhaust sometimes gives you the impression that you're going fast but you really aren't lol.