I used the bag since 6 months on Expeditions in:
Siberia, Kyrgystan, China, Pakistan and Nepal.
Extremes were: -20 degrees Bivouac outside at Muztag Ata Peak in western China.
I have tested the bag to way below zero and still found it comfortable but I was wearing polypropylene underwear during the colder times.
Especially the straps which avoid the bag from sliding off the mattress and the small packing size are a real pleasurable experience.
The zipper could use some rework (which I think has already happened). Obviously had is the bag not designed to go below -10 degrees as it is not a mummy and will let you get cold ears out there. In combination with a bivouac sack or inside a tent I am sure the Ascet does perform well way below zero.
Comfort Zone -0 Extreme Zone -10