I hate to sound like an idiot, but I don't see why you couldn't slap larger turbos on and just run them with a stock file.
Regardless of what the turbos are capable of the car should limit the boost to whatever the ecu tells it to.
Just a very basic hypothetical, but if you're tuned to boost at 10psi the N75 will activate your wastegates so that you only see 10psi. If you put a larger turbo on you'll have more lag but 10psi is still 10psi, right?
That is kinda what I was thinking, I know its a did car but my eclipse I put a 16gturbo on my car that had a built motor and waited to tune till I could afford DSM LINK. The car ran the same until I started tweaking with it.