Sorry, not much help here. I played with synthetic bags over the last year (a Thermarest quilt, and a Thermarest 20 or 30 degree bag, I forget which.) I never got to push the limit on the bag, so I can’t really give you reliable information. However, the 40 degree nights I spent in it, it worked just fine, and the build quality was good.

Having said that, I think it’s also relevant that after my little experiment, I went back to my Thermarest down quilt and bag. I do have experience with both of them. For some reason, I’m not sure they list the actual EN ratings anymore, but they did when I bought them, and the 30-degree Comfort rating was dead on. Very nice build quality - 650-fill down, so they didn’t break the bank, either. (Found both on sale.)

Pay attention to the EN ratings; they appear to allow accurate comparisons. As far as brands, what about Kelty, Big Agnes, or Sierra Designs?