[steve00s]

There aren't many more than 5 things to do to increase HP short of engine conversion or forced induction. If your very first thought after purchase is dramatically increasing HP than this may not be the car for you. You can achieve some noticeable gains by:
1) cold air intake
2) some form of modified exhaust (be wary of side effects such as resonance)
3) ECU tuning
Pretty standard recipe.

I have gained just as much or more pleasure from the short shifter and the suspension upgrades. I have also found that the potential in these cars is further realized the better driver you become. I have had better drivers race my car and I am humbled. All the HP in the world wouldn't help me keep up with these guys.
Lots of luck to you.