I don't have a Cat, Resonator, or a Rear 02 and i love the way my car sounds. There is a little bit drone at certain RPM's though. I have a glass-pack for resonator but to get a full effect a person should go 20" or longer for a glass-pcak replacement.

You can turn the code for the O2 off with any tuner. HP or DHP.

Do it all!

And for what it's worth I have Magnaflow Stainless mufflers and i wouldn't change them for anything.