WeiserBucks-
I’m guessing your synthetic bag was a Slick bag (my guess could be wrong). I have a 20 degree Slick bag in which I’ve froze my ass off in 35 degree temps. I bought a 15 degree WM Badger microfiber bag after that and can attest that it’s MUCH MORE than 5 degrees warmer than the 20 degree Slick.
I woke up to a 9 degree predawn morning this November, toasty warm and comfy in my Badger. This, despite the fact that my bag was completely covered with ice, except where it was covered with water in places from the massive amount of condensation in my Hilleberg Nallo tent… I was wearing nothing but a lightweight fleece beanie, plus light socks, and light longjohn bottoms and tops, all merino. The Western Mountaineering bags are for real.