GS, O.K., so you do not believe the posts by Charlie Jennings and seem most concerned about people being bumped off in ...the backcountry..., due to believing in a situation which I find quite credible.
So, may I enquire as to your actual cold weather experience, say, how many years of regular winter sub-zero camping you have done? Have you camped in Alaska, the Yukon, B.C., the N.W.T. or other regions with very cold weather and remote wilderness?
My own coldest night was only -41*F and in an original Marmot Mtn. Works bag, rated by them, to -25*F. This was in January, 1978, in "Kokanee Glacier Park", near Nelson, B.C. and I found this bag, in my Early Winter's GT bivy and in my Fjallraven Expedition tent, warm enough to sleep comfortably....but, could have used a better pad system.
I often slept in my tent(s) at -10*F to -25*F, measured with my R.E.I. Taylor thermometer, which I still possess, ,in my old Caravan double expedition bag from Sweden and my Paul Petzoldt Wilderness Equipment Polarguard expediton bag. The MMW GT-down bag was much better than these and kept me comfy from 1978 to 1999, when it was stolen in a home B&E.
I also slept in my FF summer bag, rated to 40*F, in a tent, at a measured 10*F and in comfort on a thick foamy. This was at Crawford Creek, B.C., in Oct. 2001, when Elk hunting. I sleep warmer than most and find my VSB about ideal for almost all contemporary B.C. winter conditions....I would not hesitate to use it at -20.
My first "winter" overnight was in November, 1964 and I have spent 100s of nights in bags in cold weather, here in B.C. Everyone is different and a tent can make a BIG difference, so, I believe what Charlie is saying here.