When I ride in temps in the 20s and 30s, I add toe warmers to the mix. My cycling shoes have just enough room in them for my normal (admittedly lightweight) SmartWool socks plus a toe warmer stuck to the bottom of my feet. I cover my shoes with Pearl Izumi AmFib shoe covers. I suspect that your neoprene covers should provide the same level of insulation. The difference is really the toe warmers.

I get mine from REI - Heat Treat Toe Warmer. There are other brands out there too, but these work for me so I haven't looked any further. There is a 20% discount if you buy 10 or more pairs. And since one of my co-workers also insists on riding in cold weather, we usually buy 10 at a time & split them.

--- Denise