After spending shed loads on complete exhaust systems, induction kit, dump valve, boost controller, I'm now having a TEK 3.

It's said to run with 1.25/1.28 bar boost with standard turbo for MY02 WRx, but upto 1.35 bar for the STi.

If I fit a bigger turbo (same size or bigger than the STi), what's the max boost I can use with the standard fuel system or will a higher pressure pump be required? Any ideas?

Boost capability depends on many factors:

Turbo
Intercooler
Engine internals
Exhaust
Clutch and gearbox issues
Fuel type
Typical usage ?track
Fuel pump
Fuel pressure regulator type and setting
Injector size
Intended richness of fuel mixture
Where abouts in the rev range you want the boost
What gauges you have to monitor
Whether you are using water injection

Suggest you discuss with your mapper what you are wanting to achieve and take things from there.

Thanks John. I think I'll have to get the car re-mapped on the rollers when I fit the bigger turbo. The car already has a complete non-standard exhaust system (inc manifold), induction kit, dump valve and will have larger intercooler/waterspray shortly. ScoobySport will be sending me the SSTEK3 map for 'highly' modified cars as soon as they read the email I sent today.