In 'The Dam Busters' by Paul Brickhall he comments at Barnes Wallis was trying to develop big bombs to create earthquake effects & that he calculated the bombs needed to be dropped from 40,000ft to reach the necessary velocity. This was very early in the war and he realized he'd have to develop a bomber (the 'Victory Bomber') to get a bomb that high. In the end he settled on bigger bombs dropped from a lower height by the Lancaster. The Tallboy first due to the difficulty of making the Grand Slam. I know in that book it mentions that the Grand Slam bombs had to withstand impact speeds of about 1000mph.
I'll try to dig it out later today to see if / how it mentions how these velocities were arrived at. I thought that falling bodies would reach a 'terminal velocity' and wouldn't be that great a speed though?