No downsides of soaking feed that I know of. Beet pulp (either pellets or shreds) should be soaked regardless to prevent choke. I prefer to soak any forage pellets to decrease the risk of the horse choking. IMO, beet pulp pellets are the better deal in the long run. You get much more in a bag of pellets than you do shreds. One bag of BP pellets lasts about 2-3x's longer than the shreds. We had an older mare (30+ yrs old) that we fed a soaked alfalfa pellet/beet pulp pellet mix. It helped her gain weight and keep her weight up through the winter.
As far as other complete feeds go, when you are trying to put weight on a horse the protein content is not what you need to be looking for. A higher protein content will just make your horse hotter. A higher fat content is what you need to be looking for to put weight on a horse. Senior feed is pretty good for putting weight on a horse without making them hot.
Other than that, plenty of free-choice quality grass hay (especially in the winter) will help with putting on weight.
just like say said...no down sides to soaking feed or beet pulp. it helps to cut down on choke, and during the winter, if your horse isn't a really good drinker, it helps to get added water into them.
i think beet pulp pellets vs beet pulp shreds are a personal preference. i like the convenience of the shreds. the shreds don't take as long to soak. shreds take 15 minutes in hot water (its warm by the time you feed) and pellets take longer...i can't remember how much longer because i haven't fed them in so long. but beet pulp pellets do make a better purchase, economically speaking. but which ever way you go, you should avoid molasses. molasses can (and does) make some horses hot...and if your horse is one of those who gets hot from molasses, it will make your job that much harder.
yes, you want to avoid feeding higher protein, while trying to rebuild weight. high fat/high fiber is best, along with free fed, good quality hay. the purina senior is good for fat and fiber, but ultium surpasses it with much higher fat and fiber. that said, you will pay about $3 more (give or take) for the ultium. but you will feed a bit less of the ultium vs the senior, so to me it works out.
when i start a new feed, and i have beet pulp, i like to feed a lower amount of the beet pulp, until i can see how the horse reacts. i have to say, thankfully, no matter what purina feed that i have fed, my horses (arabs and part arab) have never gotten hot, crazy, strung out, etc, while on purina feeds. i have fed other feeds, and they were pretty wound up. when i know how the feed is working, behavior wise, i will watch for body changes. dropped weight, etc. and adjust my beet pulp from there. but i always start out on 2 or 3 large handfuls (double handed handfuls), of shreds and go from there.
i do know that some people say that they will not feed beet pulp, in any form, because beet pulp can cause blockages. i guess it could happen. i had a horse who had a bladder stone. anything is possible! so if you worry about blockage, you can add a cup of oil to the dry beet pulp. when it's completely absorbed, then you can add water to soak. the oil helps to "lube" things up and keep thing moving.