Doesn't quite work that way.
Why do you think cars make more power on ethanol/race fuels? Higher resistance to knock allowing you to put more timing into the engine and the cooling effect of ethanol.
Have a look at all the cars running E85 on these forums, all of them make more power than what a turbo is simply "rated" at. The rating is just a rough calculation from the airflow the turbo can push out.
Perfect example is Hamish's HKS 2835 (rated at a bit more than the GT-RS) as seen in the first dyno graph posted on the first page, 320rwkw on E85. That punches way above its 400PS rating.