BEP- your TB sounds like a tough keeper. You're sure there are no ulcers or other issues? OUCH, you must have quite the feed bill! But you're right, some horses just don't fit the 'average' by any stretch.
I'm pretty sure she's fine inside, just needs a very high intake. I tend to use coat condition as a guide and hers is magnificent [it was feral when I got her]. She's up to date on worming, just finished a course of probiotics, has been on a special feed to try to bring her gut PH to a more neutral level [she STANK when I got her, which is a sign of ulcers, but she doesn't any more]. Teeth are fine, they were seen to about two months after I got her [so 3 months ago]. She doesn't act or smell ulcery any more.
When I first got her she didn't want her hay AT ALL and took ages to touch her grain. She was an incredibly cautious eater, highly reluctant to try anything new... but anything hard was fair game as a chew toy. She dropped nearly 50kg in a few days, out of stress and refusal to eat, but chewed on anything and everything. [bearing in mind she's only a 450kg horse - very slender - so 50kg is a LOT of weight to lose] ...she has since put this weight back on and a wee bit more.
She's still a cautious eater but isn't picky - when I was still buting her for her leg [44 days ago she stuck it through a fence and lacerated the extensor tendon] I didn't have to do anything other than just stick the bute in her feed. She'll eat anything so long as you can get her to try it the first time.
Magic's quite a tall horse - while I quote her as being quite light, as confirmed by weight tape, she's nearly 16hh at the wither and easily that at the croup - but very slender. She is half the width of my 550kg 15.1hh Anglo Arab, and a lot leggier.
She's just trying to grow and trying to heal a leg wound at the same time, on top of being a Thoroughbred and therefore not bred to be the easiest of keepers.
Yes my feed bill is rather scary - thankfully I get my hay fairly cheap, for what it is [high quality oat hay - I only pay $90/round which is only $20 more than comparable quality grass hay], and most of her "grain" is actually legume-based, perfectly safe for the sensitive stomach AND reasonably cheap to boot. I don't know if the menu is actually balanced [for some reason FeedXL keeps refusing my credit card] but so far it's the menu she's done best on.
Honestly though the feed is the cheap part. The vet fees, the gear I've had to replace because she's broken it, the broken round pen post, the broken fence... all up, not including feed, she's thus far easily cost me over $3000 [I try not to add it up]... in five months. And I haven't even STARTED trailer training her [I don't own a trailer, so if she damages something on a trailer I borrow, I have to pay for repairs]. If I had a brain I'd be out another probably $2k in training fees, but I haven't got one of those either, so I'm mental enough to train her myself and have thus far managed to avoid medical fees for myself.
But, she's worth every cent. Every second of frustration, all the hard work, the near misses, the adjusting I've had to do from dominant pushy bossy horses to timid Miss, the re-teaching my brain so my first reaction isn't to hit her if she stands on my foot or snatches her hoof away. The worry, the stress, the heartache with her constant health issues, starting with rainscald and hopefully ending with this injured leg. The learning to control my energy levels so that I don't start off with a stressed horse before I've even DONE anything... I walk out there [sometimes after dark] and the amazing cuddles she now gives make it all worthwhile. It frankly astounds me how far she's come.