Bennjamin hit the nail on the head - best bang for buck NA mod is gearing. You can put a new clutch and light flywheel in while your box is off too.

If you've ever driven a DA9 you'd know just how crippling the DC4 non-VTEC gearbox is. The final drive is ridiculously long. I thought that mentions of 4.266:1 were errors until I drove one myself.