Did some back yard tenting last night. It was a still, foggy night with temps hanging around 30 degrees F.
I partially unzipped a sleeping bag and used it as a quilt. Had a fleece hood and balaclava for my head. Gave up after about an hour. I just couldn't get warm enough to go to sleep.
Zipped up same sleeping bag and used it as one would use a typical hooded sleeping bag. Much warmer. Was able to warm up and go to sleep in about a half hour.
I've done this experiment several times while backpacking also. Results always come out the same with temps around 30 or lower. With warmer temps I don't notice much difference because both modes are more than warm enough.
My tentative conclusion is that, for me, I get more warmth per ounce from a fully enclosed sleeping bag than I do from a quilt. I have a large bulbous hairless head and I'm guessing that it is best to keep it within the bag where its heat can be shared with the heat generated by the rest of my body. If I had a small tick-like head the quilt might work better for me.