We just spent 2 weeks in the San Juan Mountains, the latter of which was at around 9000' elevation. The temps dropped into the upper 30s/low 40s at night.
On previous trips using my synthetic-fill "30*F" bag by Wenger, I had to sleep in 3 or 4 layers: t-shirt, long-sleeve t-shirt, sweatshirt, and sometimes a coat, to go with 2 pairs of pants (1 fleece) and 2 pairs of socks. I still froze my rear-end off, at times, in similar conditions. That bag, in hindsight, sucked.
This year, we went with Kelty duck-down 3-season bags all around. The kids and I were in the
Cosmic Down 20 mummy bags; the wife was in the
Galactic 15 rectangular bag.
I started out in a t-shirt, fleece pants, and socks, and sweated the first few nights. Thinner pants rectified the sweating, and I even had to go with shorts one night. It was an amazing difference and well worth the ~$115/bag we spent on them.