I used my JRB as an UQ, a cheap ccp, and 15 deg bag into the teens and was toasty. I also sleep cold. A really easy fix is to add pad. That will give you the most bang for your buck. You could trim it down to torso length to save weight and warm you up where you need it.
I carry a sit pad that I can place under my butt or feet when I just need 10 deg more warmth in that area.