My Daughter is into the R53 super charged Mini Coopers. We are doing complete suspension and engine upgrade. One thing I can tell you. without the battery hooked up you cannot steer it to easy. When I first went to push it out of the shop I thought the steering was locked up.
So if you put a switch on the assembly , No one will ever steal your car .
Otherwise it steers great when driving it and pulls no power from the engine performance. I did not know it was electric until we started redoing the car. If you can do it I think it would be a great upgrade.