I've used my WM Megalite 30 F. bag in 15 F. weather inb Colorados' Indian Peaks range with insulated jacket and pants and found I was very comfortable. I also had thick "sleep socks" and a very warm balaclava which helped a lot.
My insulated jacket and pants were strictly "mid" layers with very light nylon shells and DuPont "Thermolite Micro" insulation, which I have found to retain its loft much better than earlier versions of Primaloft.
With this combo I also used the jacket for warmth around camp so it served two purposes, a goal of UL backpacking.
I've camped in the Dolly Sods years ago for a Ski Patrol Nordic Mountaineering course and experienced -5 F. temps overnight. It can get cold there as it is the furthest south true alpine area in the east.
P.S. See the 1st thread recommended by Tad. In it I explain why I prefer a synthetic outer bag or at least an outer down bag with DWR treated down. Double bags are heavy but they REALLY work well and that's why the military uses them.