I was going to be skeptical and say your horse is not a hard keeper if she's never been below a 5 on the body condition score. I'm still wondering because I've not seen a "hard keeper" that never dropped weight at times down to a BCS of 4. In my mind, the definition of a hard keeper is one you can't keep weight on despite feeding "the recommended amounts" or a lot more than most horses would eat.
That's beside the point. Anyway, if you're keeping weight on with 8 lbs or so (most grains weigh about 1 lb per quart from my experimentation) of the Pro Force Fuel that's not terrible, but a little risky. It would be riskier, however, to switch to oats.
I'd suggest monitoring how much oil you're feeding since Pro Force Fuel has rice bran and is high in fat, so I'd say you're over feeding fat sources. You've got BOSS (you don't say how much) which is a fat source, rice bran in the Pro Force Fuel, and you're also adding oil. Horses are best at digesting fibrous roughage rather than fats, so it's better to just add a little fat as in a total of 2 cups daily max (from all sources), and then if you need more healthy calories you could add beet pulp or alfalfa pellets.
Corn, Oats and Barley and sweet feed are ALL grains. Grains are high in NSC which is bad for some horses, but they also require the gut to become acidic in order to digest them which is bad for ALL horses. The gut can only be more acidic or less acidic. If you feed grains, the gut will be more acidic. This helps them digest the grain, but it means they digest hay less efficiently and get less calories out of their hay. Hay requires a less acidic gut for digestion. So basically by feeding lots of grain you are saying, "I choose to have my horse digest the small amount of his total diet well, and the large amount of his total diet (hay and pasture) poorly." Which creates a hard keeper.
If you can add concentrated calories with non grain products such as hay pellets, beet pulp, coppra, etc, the horse will get much more benefit from hay and pasture calories.
Why it is risky to switch to oats: Right now your feed has a small amount of corn and other grain products, so your horse only needs a slightly acidic gut to digest it. If you start feeding 8 lbs of oats, the horse's gut will become much more acidic. This can lead to ulcerations in the lining of the gut, and then the toxins inside the gut will leak into the bloodstream. The toxins go to the hooves and cause inflammation. This means instant laminitis, possibly colic, and the horse is at risk for foundering and never being usable again.