Never experienced -40 hunting but Zero degrees in a Michigan 6' x 6' deer blind for 11 hours might come close. I was layered to the hilt in pac boots, expedition polys, mid wt polys, fleece, balaclava, etc. and in a sleeping bag. Sweating and a full bladder were my enemies so I wore less layers while moving and urinated often. My theory is that a full bladder and kidneys concentrated the liquid (urine) and it chilled my lower back area...I also keep my hands in light jersey gloves then tuck them inside the bibs/coveralls and inside my pant pockets. Years ago, my grandfather used a handful of charcoal briquettes in a metal coffee can for warming. I've also skinned the deer and let it hang outside overnight where it froze solid...that was not fun processing.
What works at -40 and how long are you stationary without any supplemental heat (fire, catalytic heater, etc.)?? Without being brand specific, what is your system from head to toe? Number of layers, clothing materials (wool, fleece, etc.), and are you over-sizing the outermost layers? What supplemental heat is used if any?