I guess my question to you would be what is the cause of IR? I mean in humans, that is Type II Diabetes, which although genetic, has strong associations with diet (in particular high fructose corn syrup). Is IR in horses the consequence of a lifetime of high starch feeding?? I will check out that site.
I've been doing a lot of research into diets for horses prone to tying up, before changing my boys diet. Latest research seems to show a link between IR horses, Cushings, laminitis, tying up and other similar issues and the NSC levels in feeds (Non Structural Carbohydrates (I think)). Studies have been done and a healthy level of NSC in feeds for horses is < 12%. Most feeds advertised as suitable for IR horses, etc have NCS levels of anywhere up to 50%. There is a big push for NSC levels to be mandatory in feed labelling to horse owners can more accurately choose feeds suitable for IR, Cushings, tying up and laminitic horses. This article
has the graph showing the levels of NSC in a few Australian horse feeds marketed as 'cool' or suitable for laminitic, IR, Cushings of tying up horses. Very interesting. An article
that has a chart showing the rough NSC levels of different hays and grains commonly fed to horses.
As a result of my research I have swapped my boy from a mixed sweet-feed to a diet of Speedi-Beet, Copra meal, and white chaff. He has only been on it for a week, so time will tell if it will be a benefit, but my guess is yes.