Start with the obvious. Change all the fuel filters. Start running a 5w-40 sythetic oil (Valvoline or Mobile 1). And change all the air filters. If the stock filters are going to run 100 bucks. Just put in some AirAids and then you can clean them instead of replace them.
sorry on the exhaust advice, at least ditch the cat or muffler to let it breath better. aftermarket mufflers are always available, plus not to mention what PSD already said.