Bulk can matter as much for some Nigel, certainly if you're going scrambling. Not sure that matters to the OP however.
oh yeah I know that, I have the same volume focus on bike touring, but then it also comes with no weight focus on the bike. My bike touringvolume is 26+16=42L but I'm growing my bike capacity by a few L as there's little room for food in 42L. I had an earlier bike capacity version of 40+16=56L which was ample, so by trial+error the 45L type would do me "real world".I've formed a similar view with backpacking which is 40L tad too tight a 50Lshould be ample.
Extrapolating from my Drishell 800fp down sleeping bag, I'm sure a 6L-8L synthetic down bag is possible. Primaloft One is roughtly double the weight and volume for the same insulation as down. So if say my 600g total bag with Drishell and 300g of down became a top-bag (no insulation for just over a 1/3rd of the circle) and the shell didn't need to be as water-proof then the 300g of down all-round becomes 400g of PL1 upper-only and the shell became say 150g Pertex, you'd end up in the 550g total weight and about 7L, roughly.
The issue is such a bag is more fiddly, and likely cost more than down, and not last as long as down especially if being squished into a 25L pack. There would be also even more risk if the airbed were to puncture. You could mitigate with an external foam rollmat but then its not strictly 25L. Whichever way you look at it the challenge of the OP is finding such a bag and surviving unexpectedly bad weather and then the longevitity.
