BAR-Honda actually used an adapted F1 car to hit 400 kmph on the salt flats last year. I'm guessing with the right gearing there's not really any reason for them not to be able to go faster than that.![]()
F1 cars aren't designed for flat out, straight line speed. The aerodynamics are designed and optimised to provide substantial downforce on race tracks with corners.
Making a car go fast in a straight line isn't all that difficult, making a car that is usable at speed is much more so, and that's why the Veyron doesn't appear to be very good around track.