I too am of the "summer=snaps" and "winter=sewn"
point of view.
My feet seldom get cold, and I love to ventilate by opening up my TQ. That said, a Winter Burrow IMO should have a sewn footbox for good sealing. As was already noted you never get the drawcord completely sealed.
Muskrat, you are thinking along the same lines as I. As soon as I received my 45-50 Burrow I immediately slipped it inside my 3 Season Burrow to see how it lined up. Sure, a few ounces heavier than a dedicated winter TQ, but functional. Sort of reminiscent of the Stephenson's WarmLite bags.