When things get too cold for my trail runners with GoreTex and wool socks (around 15F), I use a hybrid footwear system which basically amounts to a waterproof mukluk. Starting with the outer layer:

Tingley natural rubber overboots
NEOS EVA foam insoles or Steger wool insoles (can't decide which I prefer)
Steger mukluk liners
spare wool socks to stuff into the large empty space over the foot
medium to thick wool socks
vapor barrier: oven bag: turkey size
polypro liner socks

The vapor barrier is critical because these aren't breathable at all. Without it, the insulation will wet out from sweat and not work very well.

My coldest use of these was in the backyard at 0F. I was sitting around playing with fire stoves, and my feet were toasty. On the other hand, they're a little bit sloppy-fitting for hiking. Snowshoeing seems to work ok though.