I have a Model S and a C6, and I have tracked both, so I'm well aware of the limitations. I've not had my MS back down the power on repeated 1/4 mile runs, but they weren't back-to-back-to-back. I imagine if you go bonzo on it you'll get there.
Yes the Model S backs down the power after a single lap, and of course its top speed isn't as high. I'd be surprised, though, if you managed to get the brakes really hot simply because the drive train will limit you first. That was my experience, at least.
It's still very impressive that a heavy family sedan can keep close to a C7 in the quarter mile. And it would win in the 1/8th mile.
Game over when the C7 Z06 comes out, though. Reportedly it will have 620 HP and 650 torque. Ridiculous!