I don't know about the exactly velocity reached by either the Tallboy or the Grand Slam, but both had offset fins to give them gyroscopic stability to counteract the effect of transonic shock waves forming and disturbing the path of the bomb.
The Grand Slam had offset fins from the first one, but the Tallboy was not fitted with them until after the first drop trials because it was affected in this way. This information came from Paul Brickhill's 617 Squadron book.
Very high accuracy was being demanded, the SABS bomb sight needed all sorts of corrections applied resulting in very high crew workload and having an errant bomb would have negated all that work put in to ensure accuracy.