I have a Nemo Disco 15 (rated -10C). Not the most super rated bag but it has features I like and weighs about 680grams. Modified mummy with some added features (vents, stuff space for ‘pillow’, )
Consider not just the bag but also the liner, the clothes, the pad and tent or other domicile.
Note: This is not a bag, used by it self, for VERY cold temps.
I just finished a few nights at 16-18,000 feet in wind and -C temps. Using a thin therma-rest pad and a silk liner and sleeping with lots of clothes (but never a puffy) on the coldest nights I was almost always warm and comfortable. I am a warm sleeper.
On ‘warmer nights’ (getting closer to 0C) I don’t use the liner and I end up removing clothes and opening vents.