This is my experience too. I am not certain how to quantify the difference between, say, an 84 IQ hitting coach and a 94 one (assuming similar patience and other ratings, which is perhaps part of the problem: most coaches feature different plus and minuses across the various ratings), but I have certainly noticed the difference between a 52 fielding IQ coach (almost no fielding development) versus a 70+ fielding IQ coach, and the difference between an elite minor league coach and a poor one. Presumably the higher the better, but again I do not have a solid enough grasp on how the various coach ratings interact with your player's own ratings (makeup being the big one) to quantify the difference between two good coaches.