The only things I can think of on a non-turbo are:
Air Filter
Exhaust
FSE Power Boost Valve
Other than that you'd be talking big money - i.e. bolting on a turbo.
I fitted the FSE valve to my old 9000XS 2.0i - though didn't notice any performance difference...
Abbott/Hirsch etc do concentrate on the Turbo's as they are a lot easier to upgrade.
To be brutally honest, if you want more power, it would be cheaper to change to a turbo car. The LPT's are 150BHP as standard, but can be chipped (via ECU) to 190BHP. FPT's can be sorted too, though at 200BHP, you'd notice the difference against a non turbo 2.3.
Can anyone else suggest any other mods that can be done relatively cheaply to a non turbo 2.3??
Hope this helps.