Best horsepower mods and I've almost tried them all:
1. Headers + High flow cat (Toda/J's racing/Maximworks)
2. ECU retune (Greddy E-manage, Haltech Interceptor, Hondata)
3. Intake (Airbox/CAI) - Mugen, Injen, J's Racing, Gruppe M
4. Lightweight flywheel - Best engine response mod if you have a manual
5. Lightweight pulleys - Helps the engine rev much faster and smoother
6. Lightweight wheels - Improved acceleration off the mark and braking
7. Supercharger - very difficult to install without ECU issues in Australia (Comptech sells a supercharger kit in the US)
8. Turbocharger - even harder than supercharging, custom kit required
9. Exhaust - only useful with headers installed, minimal gain in hp