Wheat bran is not as good for horses as people once thought. I would switch her to rice bran or flax, for added fat and calories. Be sure to get stabilized rice bran or flax, for a balanced Ca:P ratio and added preservatives (to extend the shelf life). (NVMD, I see that you feed the Max-E-Glo. Just nix the wheat bran completely)
You should also not soak beet pulp more than 4 hours, as you risk it going rancid (especially in warm weather). Shredded beet pulp only needs to be soaked 15-30 minutes before freeding. And, having to soak beet pulp is a myth. It will not swell up in the stomach or throat, and only horses that are prone to choke in general will choke on it. I have fed both pelleted and shredded BP dry without any problems.
As long as she's gaining weight nicely, then just stick to what you're doing. Once she's up to a good weight, take out the beet pulp and corn oil for a couple of weeks. If she stays at a good weight, then leave her diet as-is. If she gains, take out the barley and reduce your grain. You just need to play with things until you figure out what works for her.
Personally, I don't like feeding horse feeds or grains. My horses do better on a more natural forage based diet that is grain-free. I use my own mix of Alfalfa pellets, flax, and a custom vitamin supplement with added amino acids from horsetech.com. For weight gain, I feed 1-2 scoops of alfalfa pellets (3qt scoops) and 1 scoop of plain whole oats, along with my flax and vitamin supplement. We feed free choice mixed grass hay and limited pasture grazzing. I find that I can feed a lot less feed this way. Even my "hard keeper" only gets 1/2 scoop of alfalfa pellets a day, with nothing extra, and he stays nice and "plump"!!