6 layer vapour barrier footwear system
It took me quite a few years to learn that there are two reasons why my toes were always cold when cycling with feltpak boots (that are rated down to -40C).
They aren't breathable (the bottom section is rubber). Therefore, water vapor from my feet was condensing in my wool socks and the felt liner in the boot, so their insulation value was diminished. Applying antiperspirant to my feet helped a bit, but it also caused dry, flaky, cracked foot skin.
You have to use platform pedals when wearing big boots. The resulting push-down-only pedal stroke and flexible boot sole cut blood circulation at the balls of my feet.
I concluded that a stiff-soled cycling shoe with clipless pedals was the best way to keep my toes warm. Thanks to many tips I read about and some of my own ideas, I now ride with a 6-layer footwear system. I haven't yet found my temperature limit; my toes have survived a number of 45 minute commutes at temps below -40C.
Layer 1: A very thin nylon or polypro sock (for comfort).
Layer 2: A very thin plastic bag -- a fruit or veggie bag. This provides a vapor barrier to keep warm moist perspiration next to my feet, and keep layers 3 to 6 dry. Yes, the thin polypro sock does gets wet. See www.warmlite.com/vapor-barrier or do a website search on "vapor barrier clothing" to learn how vapor barriers work.
Layer 3: A thick insulating sock.
Layer 4: An over-sized cycling shoe to accommodate layers 1 to 3. My shoe is two EU sizes larger than my summer cycling shoe. I replaced the stock insole with a foil-topped felt one. "Winter cycling shoes" are available.
Layer 5: A thick neoprene bootie for more insulation and wind protection. These may hinder the recessed cleat on some shoes from clipping into the pedal (but not mine).
Layer 6: Size XL shoe covers that slip over the bootie. They don't have a sole, so they don't further hinder the cleat from clipping into the pedal.
At milder winter temperatures, I'll shed a layer or two.