I've had two feed changes that resulted in major, noticeable change:
The first was when I changed from a vit/min supplement (Dynamite) to a ration balancer (Triple Crown 30% Supplement). My horse's hooves had barely been growing, which was a problem because my farrier was trying to correct them, and that's hard to do if they don't grow! There was a noticeable change in growth rate before I had even worked up to the full daily amount (a couple weeks.) Later when I discovered FeedXL.com, I found that the vit/min supplement didn't have nearly enough copper/iron/zinc, which are very important for hoof growth. I've continued to be very happy with how my horse has responded to TC30!
The second was for his seasonal allergies. He developed a cough in the spring, which didn't improve with the first supplement I tried (Ani-Hist). It got pretty bad any time he did more than a brisk walk. Within a few days of starting him on Cough Free his cough was all but gone. The only problem is that it smells (and presumably tastes) awful. Eventually my horse went off his grain and I had to start mixing it in to a mash to cover the taste. Now that he doesn't need it, I rub the old bag that it came in on his lead rope to keep him from chewing on it
The other supplements I have him on are things that I wouldn't necessarily see a difference with- he gets MSM as a preventive joint supplement, and flax seed for omega-3's since he doesn't get a lot of fresh grass.