now that I have some time, here's a full writeup of my experience. Keep in mind that I'm a *very* warm sleeper, especially when I've just at a decent sized meal.
For our cold night we were in a lean-to, with 3 wall draft protection and not a lot of wind that night, but every so often you could feel a breeze.
For pads I was using a Z-Lite Small and a ProLite 3 Small with a slight overlap where my hips are. I also have my therm-a-rest pillow since I'm a side sleeper. My bottom facing side was never cold for longer than a few minutes when I would "toss" around. I would prefer having better R-rated full length padding for these temperatures. A Exped Downmat, Kooka Bay, etc., or even a NeoAir with a mylar sheet may be enough for me?
We got to 'bed' by around 7pm, and the temperature was already around -5F iirc, perhaps colder. We knew it would be a cold night so we boiled water for our nalgene's. I went to bed continuing to wear all my clothes from hiking + my down jacket (base layers, FA Hangfire hoodie, hiking pants, rain pants, down booties).
About 2 hrs into sleeping I woke up *very* warm and proceeded to take everything off except my base layers. I fell asleep for what felt like a very long time (2 hrs?) until I woke up and noticed my feet were getting chilly and proceeded to put my down booties back on, this may have been because I couldn't place the nalgene near my femoral artery while sleeping on my side(?). A good time later I woke up again a little chilly, noticing my nalgene was no longer "warm"(body temperature I guess at this point?) and put on my FA Hangfire hoodie. The rest of the night I was very comfy.
I had the face hole opening pulled tight to just over enough room to have my mouth and nose sticking out for breathing. Loved breathing the brisk air!
When we woke up around 6am our thermometer read -18F, so I assume at some point in the night it had to be even colder? Either way, for 1558g of total weight (3.43lb) for the combination, to know we woke up at -18F and that I slept very well is quite comforting.
I don't know if I'd bother with a single bag at a lower rating for a few reasons: I don't know how often I'll be sleeping in these extreme temperatures, the cost of getting a 0 or -10 down bag is pricey, I'd rather spend the money on items that have dual use for this situation like down pants that I can also wear in camp. I can take both bag and quilt and know that if the temperature is not as low as I can always take the quilt off to prevent overheating.
I believe the swing in comfort from being very warm to slightly chilly was due to the temporary artificial warmth provided by the nalgene. I'd like to try again without starting with the nalgene and having only up to my mid-layers on to see how my temperature goes over the course of the night. How often I'll be sleeping at -20F though remains to be seen, and I can always have my stove set up to quickly boil water in the middle of the night if needed.