I wear size 41 Sidis. I bought size 42 winter Sidis - too loose. I'm okay down to about 28-30 deg F, after that it's not just hands/feet.
Many riders in the area use the electric insoles available for skiing. They're good with normal shoes down to 10-15 deg (and obviously you need more stuff for your face etc). The electric insoles cost about as much as street price on winter Sidis. I'd get the electrics, use booties, and spend the rest of my money on the warmest tights (esp above the thigh), face mask things, warm gloves (lobster type), and neck protection.
I've given up riding regularly in really cold weather. If the mood hits me I'll go out on the mountain bike (slower so less wind, more rolling resistance so more work). I'm okay down to 20 deg F on the mtb without electrics or winter shoes.