I don't cycle in the cold (living here in SO Cal) but I do camp often in the snow and cold (anyone camp at Gila Cliff Dwellings NM xmas week? They have great campsites.) That old camping adage may come into play here "If your feet are cold, cover your head." 1/5 of the the blood in your body is in your head, and it is a major location of heat loss in the cold. So while it is a very good idea to size up your shoes, and fill the space with warmers and socks, and use the outer coverings, it is also a good idea to look at the whole body, and don't forget to keep heat loss to a minimum from the 'ol noggin.