I recently switched feeds(almost 4 months ago) to Strategy gx, I have seen a huge improvement in my horses coats, mane/tails and feet. More so even with my gray mare who has horrible feet I also put her on Horseshoers Secret. Her feet are growing( I can't wait until they get long enough to where the shoer can nail to the new, better growth) and her mane and tail have grown. It is no thicker really but I think the hairs may be stronger and not breaking off and in turn making for a longer mane/tail.
I agree with mls that genetics play a huge roll in mane length/thickness. But proper nutrition will help as well. I Love MTG, but it is handy for one that rubbed his mane out in the fence due to skin irritation. I would suggest doing a patch test when using MTG. I have had a couple of horses not agree with it.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.