Firstly, you shouldn't be feeding extra hard feed for the sake of it. While it's not the best pic for a condition critique, your horse sure doesn't look like he needs any extra calories, so barring *appropriate* nutritional supp to balance the pasture/hay he's on, I wouldn't be thinking of feeding extra.
Grain isn't a very nutritious feed(apart from generally high in phosphorus) and comes with other potential probs, so I generally think there are better options, for nutrition or otherwise.
I'd personally be feeding nothing but a nutritional supp, in as low a dose as possible, to avoid extra calories. You can find palatable powdered supps, or if you choose a pelleted form, something like KER Gold only needs to be fed at approx 70gms (approx one double handful) daily. I think the 'SmartPak' products seem generally good & low dose too, but they're not available over here, so only know what I've seen online.
Afraid I found the info on that product link a little confusing - talks about different prods than the one in question, the 'estimated sugar/starch' link gives info on a different product... can't find ingreds list for that particular one. It appears to be a 'ration balancer' type feed, but seems to be a highish energy/fat product that's fed in substantial quantity. Therefore, assuming the nutritional balance is appropriate for a specific horse's diet & they need extra energy/calories, then it may be OK.
Can't comment on nutritional balance - you need to do a pasture/hay analysis, or at least consult someone who is familiar with your local pasture/soil, to give you an idea on what's in it, in order to work out the most appropriate supps for your horse. Eg. Iron, calcium & sodium are commonly way high in many soil types/pastures, so you would be therefore looking for supps that don't have these minerals added, and as excesses upset the availability/balance of other minerals, you'd want to avoid feeds high in these minerals(eg. Alfalfa is very high in calcium, among others) & ensure the horse had adequate magnesium & so on. Despite company advertising, there is never a 'one size fits all' product.