There are virtually no gains to be had with just an exhaust, or even a full cat back. I have seen the dyno sheets before and I think it may have gained like 3HP with one system, and another actually net a very small loss.

It is pretty much based on your ear, if you want it to sound a certain way there are several completely different sounds you can go for, but not much in the way of performance unless you have an SRT and you can get someone to make you headers that actually fit