Other than looks, the size of the brakes may not be the limiting factor in stopping distance. Can the car lock it's brakes? If so, tire traction is a limiting factor and not the brakes. I have never braked a Model S in a racing environment so I can't really speculate on this. I'm sure lolachampcar has much more experience than I do, but the sizing of brakes also considers other factors such as duty cycle - larger brakes are able to absorb more heat and are able to stop consistently on a repeated basis (as in racing). However, larger rotors also add rotational inertia, which is the enemy of efficiency. As lolachampcar alludes to, pad material is probably a better modification than physically larger brakes (or ceramic brakes, which in many cases brake worse than standard ones when cold).