I haven't changed exhausts on my V8's except my Clubby already had XForce and I damaged the rear pipe and I had to replace as catback, I just replaced with XForce again. From my understanding a catback only isn't going to give you much if any power increase, but replacing the exhaust manifolds with extractor type should give a better power increase. So I'm thinking if the OP wants to limit the spend right now a question would be are you looking for best power increase given the dollars, or is the priority a better exhaust note? If it's exhaust note priority then given the OP has an auto (AFM) what is a good sounding catback for an AFM car (no doubt there have been many threads that have already posed the same question).