It's been mentioned on here that the Borla XR1 race mufflers sound and perform nearly the same as the ATR Pit Bulls.
They are made w/ 12" & 16" case lengths.
The Vibrant Ultra Quiet Resonator is said to be a good choice as well, but only have a 10" long case. I've run one on another car and they are quieter than you'd think. Not as quiet as a 4.5 x 9.75 Hooker Maxflow muffler, but close with a deeper tone. Just to give you an idea.