How much HP increase can a STI engine have without upgrading the turbine/compressor and internals?

Boost controller, exhaust, management, etc, I'd figure on getting at least 50 more horses out of it. At least.

Are you including the headers, downpipes, high flow cat and bigger intercooler?

I wasn't thinking headers, but yes on the downpipe and intercooler and catalytic converter. I guess headers would net you even more.

the motor can take alot of hp
people runnin fp greens w\ supporting fuel mods
and its still runniin w\ no probs
its a strong motor.
i say 450-500 flywheel