Time lapse measurments of the relative speed are about as good as you are going to get. Remember that the sides of the tank, and most of the shuttle, if not all of it, are inside the shock cone from the nose of the external tank. So the relative wind is not blowing as fast as the stack is asending. In doing the time lapse estimates one would need to take into account that the foam and the shuttle are accelerating with respect to one another, so an average wouldn't be such a good estimate of impact velocity. Double the average speed would be right, assuming constant relative acceleration, which probably isn't too far off, but one could check that by comparing the average speed over each frame to that of other frames as well as the overall average.