The OOTP manual says that hitting/pitching coaches do affect the results of the team. Here:
https://manuals.ootpdevelopments.com...ersonnel_roles
How much of an effect is hard to say, it may be an interesting thing on which to run tests. But in a 162 game season every little advantage helps.
Personally I plan to disable the coaching system from all of my saves going forward, as I think it needs a lot of improvement.