I carry the Marmot Quasar as my winter rest-stop top layer. It's similar in weight to the Montbell Smoke. Probably 3 ounces of 900 fill down. I've worn it over a Capilene 4 baselayer stopped in 11 degree (F) temperatures. No problem at all, although I was not sitting for hours and hours.
Around camp at 20 degrees, moving around, cooking, setting up a shelter. No problem. Sitting for hours at a time? Probably need to layer it or get in a sleeping bag. If I were going to be standing around (not generating heat) for hours at a time at 20 degrees, I'd grab a warmer down jacket. Or, for max flexibilty, layer it with my Zeus jacket. That pair is VERY warm and the Zeus is big enough to layer over the trim cut Quasar.
These ultra light down hoodys are fantastic because they weigh so little and pack so small, that you can always carry them and have a very warm option for rest stops or for emergency use when you probably wouldn't need it otherwise. But, the tradeoff for being something that you carry and use all the time is that you are giving up the ultimate warmth of a heavy down parka. F