Did a few tests last winter on sleep systems with another BPL member, temps were down to -20F camped on snow in a floorless Hex.
Was using a zero degree 725 fill bag, VBL and wearing a down sweater, Primalid, wool tights and softshell pants, wool base top, stretch nylon top, gloves
Foam mattress and Prolite 4 full length pad.
Partner was using a 32 degree down bag inside a BPL 180 UL quilt, 2 foam pads, Thermolight emergency bivy, wearing Cocoon top and pants, homemade vest made out of Thermolight material as a VBL top. Mid weight wool base layer and wind shirt.
Brian slept chilled, but survived the night.
Guessing inside temperatures of the tent probably were closer to -10 at the coldest point in the evening, we ate a substantial amount of calories and consumed warm drinks before retiring.
After discussing gear choices Brian's system would have been more comfortable around zero for him, but he survived the night, thinking I was "just OK" right at those temperature and slept through the night.
Hope this helps out