In modern passenger cars, turbo boost is electronically regulated, in other words, actually limited to a certain maximum level; this is done to produce long, flat torque curves and also to offer different performance versions.
So there is a large electronic component involved in addition to (the very nicely modeled in the game) turbo sizing thing.
I'm not sure how this electronic control could be implemented in the game, but it would be nice. You could, for example, limit your maximum torque by electronically limiting max boost under certain
rpm range - and you would get better reliability and possibly economy as a result. Or you could improve your low-down torque by allowing more boost pressure under low rpm. This is a common practice
in modern passenger cars.
This is what car companies of today are doing, and then they do a "GT" version or something, where sometimes the only
change to the engine is altering the electronic control to allow more maximum boost. This is also what tuning companies do when they sell you a "performance chip" for your turbocharged car.
edit. here is a sample power/torque curve of a modern passenger car engine with a sporty edge, the Golf GTI:
http://www.webwombat.com.au/motoring/ne ... -gti-5.JPGNotice where the torque curve goes completely flat? That's where the electronics start to cut back boost pressure and after that, limit it actively to such level that the torque
stays constant. This way the engine gets a wide operating range, strong low-end, without causing too much stress on the engine internals.