I have the mufflex muffler delete pipe. I really like it. Its a little loud but not bad.
We have mandrel bent stainless steel exhaust STOCK on these trucks. You dont really need to put a catback on, just replace the muffler and if you want, resonator deletes (Id just do the muffler first).
Also, if you really want some power, go with a set of ARH headers.
2015 Camaro 2SS/RS/1LE