I've never had cause to buy a bag for temps that low but I do have a bag from these guys, and it's good. Light, well made, quality down. Mine is also a generous size too - nice and wide so I can move inside it easily.
http://sleepingbags-cumulus.eu/uk/categories/sleeping-bags/down-sleeping-bagsTheir Alaska 1300 has a limit temp of -30C, comfort -19C for 349 Euros (£310). They can do hydrophobic down for a bit extra if you think you need it.
I'd also be careful about comfort vs limit ratings. E.g. the Ran Andes -27C rating is the limit rating, with the comfort rating being -15C. if the temps are actually likely to hit -20C then you need to be sure you're not going be cold. I tend to be cold at the limit temps but am fine somewhere between comfort and limit, but everyone is different.
I'd have thought that at temps that low, sweat is unlikely to escape from the bag as it's going to condense or freeze in outer parts of the insulation. 30C inside the bag to -20C outside is a 50C temp gradient. I wonder how that is dealt with? Drying facilities? I know artic explorers use vapour barrier liners for this reason but I imagine the tour operators would be suggesting that if it was required.
