Unfortunately, unless you really find a mod friendly dealer, not too much that can be done. I really only started to mod after the warranty was up. My local dealers are tough with mods, even the one that has a large aftermarket FRPP parts section inside. A catback is usually OK as it is downstream of the cats, lowering springs and if you stay with the stock rear diff cover you can usually get away with some gears - not too drastic ones though.