I recognize the problem, but... why not reduce the amount of power you can put down with lower grip?
If the grip is low, pace will actually be slower in the corners and not just the straights, not to mention if it's not possible to put the power of a hot motor down onto the surface, people will start tuning down the power of their motors. Spec motors and ESCs will only reduce speed on the straights, and people will strive to find any tricks to increase their acceleration and speed, making the class more expensive, not to mention you'd need to check for a lot of items that are not regulatory...