I happen to have an elephant foot, but have never used it except I took it on climbs where I thought we had a high risk of getting stuck overnight; never did. However we did spend one cold night out at 9600 feet and 32 deg F one time. Right next to a glacier. I had a medium weight down jacket and hood, wool pants, down pants, and wrapped myself up in a nylon tent. I got some sleep, but I was chilly.
I believe an elephant's foot would have been better. So I got one.
Unfortunately it weighs almost as much as my summer weight 800 gram down sleeping bag and I would much rather have the sleeping bag.
I am a warm sleeper. I am sure that makes a big difference. Also, closed up inside a two person tent with another warm body I find makes a significant difference. Sorry I'm not much help, but interesting to contemplate. My hiking buddy purchased a down "throw" (70" x 50") that weighs about a pound and is (I think) going to make sleeping bag out of it. That would be near armpit high (50").
Jim M