I went backpacking two weekends ago with my Dangerbird. I had a 30° underquilt and a 40° top quilt. I didn't have a thermometer, but there was frost, so the nighttime temperature definitely got below freezing. Since I only had a 40° top quilt, I zipped up the overcover and was toasty warm all night. No frost on the inside either.
In my opinion, if you have a Dangerbird, you don't need an additional winter sock: the overcover does the same thing.