Not that I know of tbh.
The instant calmers usually contain a form of magnesium that is particularly bio available (though expensive) along with other ingredients depending on the brand - usually thickeners, colours and flavouring! The calmers you add to feed contain different forms of magnesium that are less bio available (but much cheaper so you can feed it everyday without bankrupting yourself!). Magox is the best form to add to feed as an everyday thing and is the active ingredient in most brand name calmers. Worth pointing out that it will only have a calming effect if your horse is defficient in magnesium in the first place (many are though).
If you have some time before you would want to be giving an instant calmer then I would add one to his feed and see how you go with that. I wouldn't waste your money on a branded one, they are usually very expensive for quite a small amount of active ingredient. I would get from here http://www.naturalhorsesupplies.co.uk/p/category/0802204936-Magnesium+Supplements/ super cheap, good quality and useful info on the website to help you choose what form of magnesium you would like.
