Alex, my personal love of quilts is mostly centered on this very subject. The variable girth allows them to be camellion-like in use. The more you wear to bed, the less quilt you need.
The first time I ever used a quilt, it was a 40 degree golite, brand new. A weather front moved in with cold air across the mountains of New Mexico surprising me with a low 20's night time temp. I had a warm pad, and I put on all my warm gear (not much, really), cinched everything down tight and readied myself for a long and miserable night.
I slept like a baby. I was fine. Certainly not hot, but A OK.
I think the biggest factor is what kind of backcountry fella are you? Do you climb hard in minimal gear, then bed down quickly to survive the night? Then perhaps you do need a 20 degree bag for a 30 degree night. Or, do you get to camp early and enjoy lounging with the boys by a fire in your super awesome warm down pants and parka? Then I suspect a 40 degree quilt would more than do the trick. After all, you're gonna wear that stuff to bed.