I saw the other thread that said in tests that a entire cat back system doesn't help mileage significantly.
SO, it's done more for sound and performance.
IN THAT CASE, wouldn't a weld up muffler do the same job? After all, the pipes after the cat back would be the same if changed out. Also, you have the mooola $avings.
I'm thinking that my supposition above it true in that most people change the exhaust for the sound.
So, what's a good aftermarket muffler that's not as loud as a flowmaster, but allow better exhaust and a good sound?