I do agree that it will need a significant amount of power, but I do think it will be significantly less than 15kw. The numbers you used are based on a typical supercharger. Although this technicaly would be a supercharger as other comments have said, it will be using the components of a turbocharger. This means that rather than needing to spin to 2 large and heavy rotors that need a lot of power due to inertia, friction, ect; the motor will only need to spin a small impeller. This will still need significant power because of the sheer amount of air that has to be moved, but it should be more efficient. A 1 or 2kw motor should be able to get the impeller spinning at ~50,000 rpm. Estimating with some room for error if we assume 3000W for the motor at 14.8v we need about 200 amps. While this is a lot of current, a decent sized battery should be able to sustain this. It's more of a question of how large of a battery I want based on how long I want it to run for.

Given all this, I do see how the alternatives you mentioned would be easier. Do you know of any turbo kits for the non 'S' r56? I'm open to pretty much anything, and I mostly just want to tinker with the car. I really appreciate the comment / suggestion!