Your factory exhaust is 2 1/4 inches. Most of the aftermarket exhausts are at least 2 1/2 inches, so you get better flow. You won't pick up that much extra power without going with an offroad (catless) H or X pipe. GT's can pick up as much as 15-20 horsepower with a good flowing exhaust.
You will get pretty much the same sound going with just mufflers as you would buying the cat back from the company, but if you are looking for gains, I'd recommend just buying a catback. That and you can install the catback yourself with minimal tools, a jack and jack stands, and a little time.
