Well my DD is 11yo and was clocked overhand in the fall at 47mph and can pitch 46 accurately. She can throw as fast as 51 but you never know where it might go and she can pitch up to 52 but again it's over the backstop or in the dirt most of the time.
Based on her game performance 47mph is more than enough to throw even the fastest girls around here out from third by at least a step.
The good ones will eventually get more speed pitching, than they can get overhand. It has been my experience that if they can reach their overhand throw speed, with their windmill pitch speed, they can surpass it pitching.
I spend a lot of time on the OH throw, during pitching lessons.
Most college recruiters are pleased if overhand throwing is 57mph or higher, which is somewhat slower then they look for with pitching speed. The NFCA has stats but I can't seem to find them with my sub-par google-fu.