I have used them to -20C comfortably:
in camp, with hot food. Also with booties and big belay parka.
I get cold easily when stopped, and these pants work for me.
When it is colder (-10C and lower) I throw them on over my shell pants/bibs - appears warmer to me than alone or under the shells, and the snow brushes off easily at those temps.
I would only get something warmer if I expected to be in sustained (several evenings straight) -25C or lower. Or do like Mike Clelland does and just wear a 100weight fleece layer over my baselayer all the time, using the Cocoon pants for extended stops.