Maybe that is where the impact is less transparent. If I consistently have an mlb hitting coach that has 90+ in a hitting rating, when my 19 year old is up for spring training, maybe he develops 3 points in contact and split at age 19, 2 points in contact and split when when he is age 20, and maybe 2 split and 1 contact when he is 21. However, if I consistently had a hitting coach with a 75 hitting rating, maybe the player only gains 1 or 2 points in the first spring training, 1 point in the second spring training and one point in his third spring training.
I dont know if this is true, but even if it is, i can't confirm it because all we have is the year to year development as opposed spring training development. If these high priced elite mlb coaches are having this kind of "over time" impact to young players just by spending 18 game days with them each season, it would be nice to know that. If they do have that impact just in spring training and you start to do that math (3-4 extra points of contact, 3-4 extra points of each split, 2-3 points of batting eye) over the course of the first three seasons. That can be a huge difference of 11-15 points total. I am a big believer that every point increase in contact, split and eye is a point increase in obp. If this is the case, the extra 11-15 points in OPS over a career of multiple players has a huge impact.
But again, I dont know if this is true and I cant benchmark a player against himself. I would have to find a similar player on another team that played a similar amount of ST games, but under a coach that is significantly better or worse than my coaches.