I just discovered that I can wear both my Sugoi Resistor Boties and a pair of neoprene shoe covers at the same time. This was good into the mid to upper 30s on a recent tour. Though I could still feel heat leaking out from the cleats and chilling the bottom of my foot and my toes were a bit cool. Maybe there are more insulative footbeds that would avoid some of the cleat heat sink effect? Anybody tried sole insulation?

For commuting in true winter weather, I switched my mtb/commuter to flat pedals with toe clips (no straps) and wore old hiking boots with heavy socks. That was great down to 0F for rides of a few miles.