but...
When this is running it will be less efficient than a supercharger (losses in supercharger motor and in battery charging and in alternator and alternator belt, as opposed to just belt losses).
And supercharges are less efficient than turbochargers.
FYI, you can exercise a fair amount of control over a turbo too:
Turbocharger - Wikipedia, the free encyclopedia
so with turbochargers so well established, and if you have to swap out the additional component known as the engine, then how is this remotely related to efficiency?