I've been giving this area some thought based on my FOF experience. I was never a huge fan of mentors in that game since most were past their prime and it required occupying roster spots just for those.
I would think however that if a 61/72 WR was on a team with another pair of WRs rated 92/92 in his 3rd year, and 87/89 in his 4th year that just practicing with those guys and observing them would make him better no matter how good (or bad in this case) the WR coach was.
On this topic, in a situation like this does a bad WR coach actually end up being detrimental in your mathematical modeling? Can he make a WR worse if his ratings are much lower?