I have the -30, and IMHO, it would not keep one warm at -20, without sleeping in extra heavy clothing. I'm talking Mtn. Hardwear Chugah(spelling) pants, and a Patagonia Das Parka.
The 6'6" bag is oversized though, So its great to layer in, or like another mentioned, slip another bag inside. The only bag I would trust to its rating would be a WM or simular down bag, but then of course, the longer your out, the more the down bag will gradually begin to loose its loft.