"Specifically, I am interested in your experience using warm layers for winter camping. What works for you."
I love winter camping, but being a humble grad student, can't spring the dough for a winter bag. This has not hindered me at all, however--infact I am going out on an overnighter this weekend.
The coldest I have slept out in was -15C (5F), which was last year, and that was before I owned a down sleeping bag. For that night I cut pine boughs for my first layer of ground cover, then a space blanket on top, then my Big Agnes insulated air core pad. I won't even bother to list my layers, as they are heavy stuff, but are cheap and effective. I own no down clothing, and rely only on wool and synthetics.
Went out twice last month, one night was -2C (28F) and the other was -6C (21F). The difference is that I finally got a 3 season down bag, so to save big on weight and also to take less layers, what I did was use two sleeping bags. I have a summer synthetic bag that I use as a liner for my down bag, and on both nights was able to sleep quite well, and on a pad with an R value of 3 rather than my Big Agnes, which has an R value of 4. I plan on doing the same thing, using two bags, this weekend, and temps are supposed to be between -3 and -5 C.
So if you have another sleeping bag to use inside or outside of your quilt, that might save you weight. My summer bag only weighs 476g with stuff sack, which is less weight for me than taking say another pair of wool socks and an extra sweater in my pack, plus I can wear a lighter jacket too. I should also add that my down bag is only rated to 4 degrees C (39F).