Unless your muffler is completely clogged, you aren't going to see hardly any gains from a "performance" cat-back exhaust (or components thereof). Most of the gains come from replacing the header and catalyic convertor.
It appears that muffler is comming out of an abusive relationship, it's dammaged goods. Everyone knows where it's been, what man would want it now? Nah, that's too much emotional baggage, I'll hold out for that virgin HKS exhaust that's saving it's self for me. :wink: