FWIW, this morning I used the disposable shoe covers over my socks, and the shoes sprayed with plasti-dip and it was completely satisfactory at 5°, against a 5-10 headwind. I can't say more than it worked one time, for only about 7.5 miles, since we only get this once every 10 or 20 years. And in general the rubber spray tends to degrade and rub off over time. Yet if you have only occasional need for more thermal shoes this trick really did pretty well.

Also the old ratty running shoes (I was about to throw them out) look pretty good after the treatment.