A 5-20 degree synthetic bag is fairly bulky. I think a down bag and a smallish packable tarp for rain protection would work. I used a 20 degree synthetic bag with a bivy sack on cataract last month and slept comfy, but woke up really wet every morning (condensation) so if i had down it would have sucked. But i was rafting so space was less of an issue.
If you have time to let your bag dry every morning then you should be fine with down, but i would make sure to bring some sort of rain protection even if its a tarp that you wrap around yourself in case of rain.
If you are lucky enough to be in the mountains, you are lucky enough.