Love the Air Core myself, but I tend towards warmer (non-freezing) conditions. I'm also a hammock sleeper.
When it gets down to the high 40s, I like to take a simple piece of fleece, double it over, and stick it in the sleeve of the BA sleeping bag, over the Air Core. This works great, even hanging in the air, down to the sleeping bag's rating of 40 F.
I like this better because the fleece is itself light and super useful when I'm not in the sleeping bag, which is hopefully most of the time :-)