A better air intake system I suppose or an ECU reprogramming..You could do both and get even better results!You're gonna lose your warranty though..
After that, I'd put a full exhaust system(kind of expensive) and better pistons etc
The cheapest way to get more performance is a re-map.
The second cheapest is to get re-map and induction. Before going into intercooler etc. I would look into the turbo upgrade. This also means to replace the sensors.
But sensors, induction, re-map and turbo will be about as much as replacing it with an exhaust system worth mentioning.