If you are still for a better sleeping bag I suggest you look at the newer materials out there. Some are much better than goose down and are not affected by Moisture. One company fits inside a two litre bottle and is good for -10. There are new models that even heat themselves
Better than goose down in what way?
I very much doubt that there is any bag that fits in a two liter anything that works to -10. Plenty of bags make outrageous claims, many of them are lies.
If you're meaning that synthetics stay warmer when wet - that's only partially true. Both types of bag are miserable things when truly wet through.