Very interesting thread! Around where I live, we don't have these problems. But back when I lived in colder climes, a Valandre sleeping bag kept me warm. I don't know what the rating was, but it kept me warm no matter where I was for nearly thirty years. I see that Valandre is still making expedition bags for extreme cold. They're also extremely expensive; and robust--I still use mine 30+ years later, just not for anything below 20 F anymore. Moontrail carries them in the U.S.--I don't know how the prices compare to Europe.
I'm a cold sleeper and a Valandre fan, and I'd have bought a new one if I lived in Michigan (or Sweden), but I don't need one in New Mexico. Instead, I have a Feathered Friend 0 F bag--just to give you an idea of how expensive Valandre bags are: Feathered Friends is cheaper.