The best setups in my opinion are the UltraFlow SS setup and the Magnaflow setup. The UltraFlow SS catback is $349 on Jegs and Summit. They are nice and quiet when you aren't getting on it. When you get on it, it's a bit loud though. I don't think anything is going to fix that on a Mustang. None of that 2100 rpm drone that drives you crazy on the highway though. Just did one on a Cobra with longtubes, and it sounds really good.
Kurt