This may not count, because it is rare for me to go under 20F, but it did go down into the teens couple a couple trips last year and I did it with a quilt. More specifically two quilts; a Tim Marshall Cuben inside and Arc Specialist with 2 oz overfill in the foot box. The Arc is wide enough to prevent drafts given that I am kind of skinny. Right now I am trying to complete the "Honey Do" list so I can leave tomorrow for a BP trip. If there is an interest I can post some pictures when I get back on Sunday. Theoretically this combination should be good to around 6F (3.75" down)? and with a down jacket and pants even more. I do wear an insulated balaclava, but I would bring it anyway with a sleeping bag, as I would the down clothes.
I am a desert rat and hate cold, so I work very hard to stay warm and dry. The above combo weighs less than my WM ultralight and has much more loft, at least on top of me, on the sides and at the bottom.
Even in winter I am going to try and shave every ounce if I can, and to me the bottom of the bag is un-needed weight for the places and conditions I go.
If there is any chance the temp is going to be near 0F or worse... yes to me sub zero F is worse case scenario, I just change my destination :)