They did that top speed test about a month after last season ended.
http://www.jalopnik.com/cars/bonneville/index.php
The car set the unofficial F1 speed record at 266mph. While within F1 regulations, I doubt you would ever see an F1 car set up like that one was on race day, it would be too unstable to do anything other then a straight line. Still, the F1 car is the pinnacle of racing technology, I highly doubt that much of anything could beat one on most tracks.