Bear in mind that as my Alaska trip is cancelled and I will probably now never get back to the USA and.or Canada this is hypothetical.
For my proposed winter trip to Alaska I was worried that my sleeping bag might not be warm enough so I ordered a custom overquilt from Nunatak.
That uses the very light but windproof and highly water resistant UL Robic fabric as the shell and 100GSM APEX insulation. It has a fitted footbox and a pad sleeve sized to fit a wide pad, with base layers, socks and a LW balaclava it is probably good to around 10C on its own but adds 10 degrees to a sleeping bags bottom line.
When I sold my deep winter sleeping bag I sent my old bag back to the factory for a rejuvenation and due to a mistake on the factories part it was mistakenly refilled in the bottom half only and with 150 grams of 700FP down after my complaint an extra 100 grams of 850FP down was then added to the top half. So I have a sleeping bag that weighs 1250 grams and now with 70mm of insulation; therefore rated for about -18C with a margin for safety due to the better insulation of an over stuffed bag.
This left me with a problem for warm summer weather as I had no bag suitable for the hills in the warmer months.
After much thinking and talking to people I have bought a Sea to Summit Traveller 2, although I consider the bag underfilled at the factory and it will soon be topped up with an additional 100grams of 850FP down. The S2S rating for the bag is 5C with LW clothing as per the EN standard. It will be warmer when it is topped up and conservatively then good for 0C
This bag is similar to the Feathered Friends Flicker but cut a bit larger and just a little heavier. Because of the sizing it fits as an overbag to my old winter bag except for the very top section so I simply reverse it and put the open zipper section under me., and this should add at least 20degrees to any bag underneath it.
Now because the Nunatak custom overquilt was made to go over an Arctic bag it also fits over the S2S bag.
So my theoretical system should be good for [ 0C + (10C) + (-18C)] or put another way it's -18C rating lowered by the rating of the overbag system which is about -10C or -28C in total.
The weak point would be the head area but I think that can be easily ameliorated by wearing a down hood and taking an idea from Kevin Kinney a fur ruff on the sleeping hood or LW down parka. There is now no room inside the old sleeping bag for a heavy weight down parka but a LW one will fit as will LW insulated pants.
As it stands now the S2S Traveller is a good enough and comfortable summer bag and with the addition of the synthetic overquilt good enough for snow conditions here in Oz and can be boosted with warm clothing for the extremes of winter here and I think the triple combination with light weight insulated clothing should/could cope with -40C. Not quite the -55C of the bag I sold but then those very cold temperatures are now probably something I would not attempt. The downside is that the triple combination weighs a kilo more than the single bag and takes up more space in the pack or the sled.