FWIW, I just did some resarch in the archives here and on the other mustang internet messageboard and found that the most recommended performance muffler for quieting down exhaust is the Dynomax Superturbos. People say that it'll make it just louder than stock. Most people prefer their mustangs loud and so a lot of people that had them didn't like the sound of the mufflers saying they were too quiet.
EXACTLY!!! I have them on my 96. BBK L/T's BBK o/r h pipe and Dyno Max Super Turbo cat-back. It is very quiet I love it. My car is the biggest sleeper.