This is really random, but I just wanted to say, some friends of mine have been getting these really nice fake tails for really cheap... Because they're off of cadaver horses. It sounds a little... Eh. But the tails ARE amazing, and they're cheap... Just a suggestion in case you do end up going with a fake tail.
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.
I found this nifty website that shows a different way of growing out tails. I just started using it on my horse so I dunno if it really works but I though it was worth a try, since I can't use mtg and I can't bag his tail up, he needs it to swing at flies. Lol.
Nut anyway heres the website http://www3.telus.net/sherrisstable/HorseTails.htm
Condition your manes and tails with Virgin Coconut Oil found in the vitamin isle at Walmart... It completely absorbs in the hair shaft and keeps your manes and tails complete conditioned and dirt free.