My suggestion is to call SmartPak and talk to them about this. Their people are very good, most of them seem to be horse owners themselves, and their customer service is the BEST. Give them a ring, tell them about your horse's history, see if they have any recommendations. My experience is that they don't oversell stuff.
I rang them just last week - I had an awesome fleece cooler for my horse, and it was super - easy to wash, beautiful, warm - with the major hitch that it was giving him blanket zap every time I took it off, and he turns out to be freaky about getting shocked. I tried washing it, fabric softener, you name it. Finally, I called SmartPak to ask about what kind of blanket I ought to replace it with.
Their suggestion? Maybe to try some show-sheen under the blanket, or use a mix of fabric softener and water to cut down on the static. I called them all ready to drop $$ on a new blanket, and they didn't try to sell me one. Eventually, I asked what they'd recommend if the show-sheen approach didn't work, and the woman put me on hold for a minute while she did some research, then came back with two suggestions, neither one of which was particularly expensive. No attempt at all for the hard-sell. That's been my experience with the SmartPak supplements, too. So give them a call, see what they have to say.