The actual cats are the only thing in my exhaust that's still stock because I didn't want to have to deal with them yet. I removed the back cats (the ones AFTER the o2 sensor) and everything is 2.5" all the way through the exhaust and I think it sounds great, but not that loud. Since that's what I was going for, I was happy with it. I'd be interested in hearing how yours sounds after you're done!
2012 Camaro 1LS - (sold to upgrade to v8)
2013 Camaro 1SS - (sold when my wife lost her job)
2013 Scion FR-S - current DD
2014 Camaro 1LT, Red Rock Metallic (wife's car after having a full-time for a good amount of time again