I'm thinking along the same lines as ViperZ.

I've ridden in -10 to -12c weather this Fall with just my canvas runners & 1 pair of socks (I had ran into trouble swapping pedals & have used my flat pedals for a few days). My first winter riding I used an old pair of un-lined hiking boots with up to 3 pairs of socks... it really is an individual thing. I'm not super-fast either, so I guess that helps a bit too.