MTG does work really well but I will say that I have recently seen that for the cost of it, the differences I have recently been seeing are not really that significant. Keeping the tail tangle free and protected when out in the pasture etc, is what will help. Your horse needs to be on a good diet.
My 2 babies have some nice tails. My big beast has been on MTG and her tail is quite thick. My Canadian's tail is way thicker and he didn't receive any MTG (recently) tho I am also contributing this to his breed. They tend to have really thick mane and tails.
As long as your horse is getting all the nutrients it needs and you do take the knots out of your horse's tail, you should be doing just fine.