As a followup, I slept out last night in my ENO DN with the JB with temps around 35. I used some CCF (20" X 24") for my feet and lower legs and stayed nice and warm all night. I'm a fairly cold sleeper. I did think I was suffering from hypothermia in the middle of the night
but then woke up from the dream and realized I was toasty!
Just goes to show that thinking too much about my gear is hazardous to my peace of mind!