Well, if the horse is magnesium deficient, the magnesium in Epsom Salts will go some way to making up for that. However I would think the laxative effect will most likely work against any possible benefits, if it speeds up the time it takes food to pass through the digestive tract.
Magnesium oxide is surely a much safer bet, given that you can get a much larger amount of magnesium into the horse's system before having a laxative effect.
Has your horse showed up as deficient on a blood test? Also bear in mind, adding magnesium affects the balance between magnesium and calcium, so you may need to take this into account.
I have tried magnesium based calmers in the past with no success. She is currently on Cool, Calm and Collected (which is calcium based) from Equifeast which has been very good. Vet said to save money and use Epsom Salts instead. I might file that piece of advice in "do not use" lol