You are three dietary steps ahead of most of the horse world
Gobbling her food may have started out as a hunger thing but has since developed into a bad habit, but I might still watch her for other subtle signs of I-R as time goes on Something I completely overlooked until this year is the protein source for 99.9% of the vit/min supplements on the market is soy
Soy is not good for insulin resistant horses either.
It might be worth a phone call to the 800# on your supplement container and ask them what the protein source is.
To my knowledge, there are only two vit/min supplements on the market that are soy-free; EquiPride and LinPro, but that doesn't mean there aren't more out there.
I cannot believe the difference in all four of my horses. I have one that I discovered three years ago was oat and corn intolerant, was also soy intolerant; he is the one with all sorts of dust/pollen/mold allergies.
The EquiPride has helped that intolerance to a large degree.
It has helped my Arab with his severe allergic reactions to midge fly and tick bites. Also, he no longer gets the runny bums on spring and summer grass and he doesn't need his ulcer meds anymore (he was a starving horse rescue years ago).
It has helped the two that are insulin resistant tremendously.
Here's the link to EquiPride's guaranteed analysis. EquiPride? and EquiLix? 1-800-327-9222
It is pricey, but, I have been able to either eliminate things (like the Arab's ulcer meds), or cut back on others. I have never in my life seen anything like this product and keep waiting for the other shoe to drop but so far, so good. I think EquiPride really is one of those "too good to be true" products that really works.
It also has a pre/probiotic in it, so it might help your mare with her digestive process, therefore help to slow down her eating.
When I spoke with one of the EquiPride owners, his comment was EquiPride "chews/digests the food for the horse, all the horse has to do is swallow it"