I experimented on three non successive nights last winter, sleeping in sub zero temps. I walked less than a mile before I unrolled an MEC yellow closed cell and an old 2" Thermarest. I wore a pair of MH insulated pants and a micropuff jacket, and slept without a tent or tarp. I have a synthetic overbag with 3 or 4 ounce pl sport, and on the first night I used a FF Condor that has about 20 ounces of down. I moved the down away from the center of the bottom, and during the -6F night I removed both the pants and jacket.
On the second -11F night I exchanged the Condor for our Nunatak Dual quilt, also with about 20 ounces of down. Within an hour, I had the insulated pants pushed to my thighs, and the jacket off, but around 5am, I had them back on. This was the most comfortable night.
The forecast was colder for the third night, so I switched back to the Condor. I woke up around 12:30 AM, warm and sweating, figured I'd blown the experiment, and was close to packing up and leaving. Instead I got up, peed, removed the micropuff, and went back to sleep. The temperature dropped to -23 that night, and again, around four, I had the jacket on again. When the temps were at their lowest, both shoulders were cold when I slept on my side. The top back of my shoulders were cold when I was lying on my back. This was the only night when I felt cold enough to wear my three ounce BPL hood. I should add that none of the bags has a hood, but the overbag is long enough to pull over my head. And yes, it was wet, but the tunnel shape that I tried to maintain in the overbag was relatively warm.
For this experiment,the inner bags in each of my kits were inefficient in their use of down, but in sum, I agree with Dave O. The synthetic overbag contributed more warmth than I expected and I noticed condensation only near the head of the down bags. Frost was prolific on the top of the overbag.If I was making an overbag for your situation, I would use the 5 to 6 ounce insulation Dave recommended, enclose in bag shape, with a short side zipper and an uninsulated bottom sleeve for some variation of GG Thinlight padding.
But I would be wary of the meager 10 ounces of down in your NO 3. Fifteen plus ounces would be a lot more comfortable at -10F. Can you wear insulated clothing in your Montbell? Maybe place a jacket between the bags over your torso? Maybe a vb? Will you take insulated pants?
