The idea is deleting the friction brakes entirely, saving costs, maintenance and weight.

And 400kW isn't really all that much for a sports car. I remember 911 ads from the '80s that boasted "brakes with more than 1000 horse power".

So how do you stop then if the batteries are (close to) fully charged? You'd need to shunt that power into a big resistive load, and then dump that heat.