for really cold, I like one full length WM pad with another 1/2 pad criss-crossed under my torso (shoulders to hips) this gives 1 " of ccf pad under the torso, where the max compression occurs. and seems to enfold you in a pocket around your sides.
I have to confess that I have never been out that (8deg.F) cold. One night at around 15 in a 24 degree Kelty bag iin a shelter. I "shivered the whole night through" (ref old folk song)