When he was much younger my son was coming off some arm problems and the doctor noticed that he had an internal rotation deficit. He went to a very good PT who did a great job of stretching him out and continued thereafter with sleeper stretches.
The interesting thing was that he picked up 5 mph after getting rid of the rotation deficit. Obviously one doesn't gain 5 mph just by doing sleeper stretches if there's no significant deficit but it does show that an internal rotation deficit can indirectly impact velocity.