I have a "big" BA that adore. it's quite warm, but I do sleep cold. I find it pretty good for me down to about -10C on it's own, but below that I top it with a blue foam pad. If I'm expecting serious cold (like this week) i.e. -40 - I use another blue foam pad underneath it.

Mine's the big (25" wide) square insulated Aircore though - not the mummy