FWIW.... i had a test pipe on my car, it was throaty and had the 'rifleing sound under throttle...
put my 'stock' cat back on, ALL of that was gone, and i couldnt pass smog.
get a new cat, and my rifleing sound is BACK!!!! omg wtf!?! no way right? no, its because it wasnt CLOGGED full of...