Anything to do with the engine block will void the warranty, so a cat-back with a hi-flow muffler is ok or even just the muffler. For new cars try and go as THIN as you can with the oil. Less resistance, less petrol needed and more POWER. Castrol R 0w 30, or mobile 1 would be good choices, but make sure you use FULLY SYNTHETIC OIL !! either way.
Once you have all the mods that you want installed, disconnect your battery and press the brake pedle repeatedly. This drains the car of all residual chage, resetting the car's ECU. When you reconnect the battery the ECU will start liearning from scatch taking full advantage of the new mods.