For those who were around in the '70s, about 1978 or so Mecham Pontiac in Phoenix (IIRC) built something called the "Macho TA". They ditched the muffler and let the cat do all the sound dampening. I didn't like the sound but the cat did reduce sound somewhat. Of course in the late '70s the GM cats were the pellet type, now with the monolith type it would probably sound different as well.
R.