I have wondered that myself. My guess would be about 265 but I bet aerodynamics cause issues at that speed and the car tries to fly.
I forget what its limited to in regular performance mode, I think its around 240. In top speed mode the car hydraulicly drops itself as low as it can and then it tucks its spoilers in to decrease drag. If you turn the steering wheel more then 45* or if you press the brakes the car automatically pulls itself out of "Top Speed" mode.
Hell, when you brake from high speed the spoiler flips up and acts as an air brake.
Its truley a masterpiece and sadly I will never drive one or even sit in one. Its nice to dream though. I am just awed by all the tech and what it takes to make that kind of power reliably. Damn car has TEN radiators!