I've used those mini heat pads too and they work really well.
Our weather is way to cold for toe covers... I wear the full booties.
For more extreme winter temperatures, most of the MTB riders I know own a pair of LAKE boots... lined with gore-tex and thermo-lite. I've heard those are "bomb proof". I don't have a pair... they're expensive, and I haven't been riding enough to justify them.
I've worn my Shimamo boots, (no booties), down to about 15 degrees and they were OK for about an hour.
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