How much does a good coach actually help in development of young players?

You'll notice if you have a bad coach. 

Not sure that answers your question but, if you have a bad one in your minors, look at the development(or lack thereof) of those players.

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. 

If all other ratings are equal, anyone want to throw out their guess as to the lowest acceptable IQ number by level. How much better does high patience and/or discipline matter?

I believe patience helps, but not sure about discipline. reading between the lines in the "help" section I think too high of a discipline number might take time away from the coach's primary job...