I would also like to suggest the Thermal Exhaust System. They are fully stainless steel in construction whereas a lot of other cat backs are only stainless steel mufflers with ceramic coated piping. The tip is 3.5 inches and it provides a nice, deep tone that isn't too loud. Provides a little extra punch in the high RPM's.
If you dont want loud but a very nice deep tone get the Thermal Stealth
Here's a pic
