Alright, I just put a BBK o/r X on my stang with MAC ceramic headers and flowmaster american thunders. The car sounds UNREAL. I can't stress enough how awesome the car sounds.
Don't even think about getting catted anything for aftermarket.
I replaced a set of BBK high flow cats (already spoken for, sorry fellas), and the difference is night and day in sound and performance. I can't believe how a 5.0 sounds uncorked like this. It sounds real low and nasty until about 4000 rpms then absolutely screams after that.