Feet are a *****. I've tried Lake 303 winter boots, they are less warm then I expected. I've used medium wool socks in my summer cycling shoes with neoprene booties, which are good to about 30. I just ordered some AeroTech over boots in 3XL to use on top of the neoprene booties, theory is to keep the wind off the neoprene.
Lakes theory on why their boots are so high is you need to keep the ankles warm, blood flow is very near the surface here. Makes sense. I also use a Coop thermal tight, has a light fleece layer, to keep legs warm. Also a warm hat and no joke, a cold head means cold hands and feet.