In my experiance, my legs and feet are very light as compaired to the rest of my body. Therefore that part of the body does not compress my quilt very much. ( less compression more warmth) But I do have a small sit pad of wally world foam that I insert into the foot end of my quilt if the need arises. This little pad is easy to carry in my Gossimer G4 Pack as it is fitted into the front side of the pack between the sholder straps that makes the backpack support. Something like this
I have never had an issue on that end in cold temps if I have the sit pad slid into my quilt. I like the idea of multi-purpose items--Sit pad, back pack support, and foot lower leg comfort in cold temps.
If you think a cold lower body it could be an issue in your situation, a full underquilt would be the answer.