When I go to see how Western Mountaineering does it for their deep cold SBs, I see two layers, which limits the variation in heat flow. Other steps to stop air leaks, too. Severe temps are associated with wind, and convection currents matter a lot more.
Even if this were not going to be heavy, fit matters more than it does at higher temps. I wonder if more attention needs to be paid to controlling air and water vapor with fabric selection, as against brute force of lots of down.