consider the double bag 'system'. a really light synthetic outside to cope with moisture, a good down inside.
1 + 1 = 3, lots of options, weight works out about the same. andy K has written about it, as have others.
as it goes im heading out tomorrow with just such a system that ive slept in happily to -29. weighs 1450g (650g synth/800g down, which is similar to a single bag rated to such, but deals with moisture infinitely better, especially over a few days, and i have 2 bags to use in all the other seasons.
bit of a pain in the ass till you work out how the 2 bags best work together, but really comfy when you do.