Easiest and highest non invasive bang for the buck is the Eurocharged Tune. $300 for about 25 hp. It brings these cars to life. Feels quicker, shifts more crisply, response is better (don't need a sprint booster in my opinion if you do this). All around better. And as others have said, the CAI from Needs wings, etc. coupled together will get the HP ratings up into the 250 to 265 range.

I'm happy with just the tune but I may venture down the other mods path later on.