Every time I've tried to design a system based on a half-bag, I've decided that it would require quite a bit more parka than I needed to carry for other purposes, so I haven't ever pursued the idea beyond theoretical.
This is kind of what I'm trying to figure out. At 40-45 degrees, I think I'd be carrying my MB UL Down Inner Parka @8.5oz, so combined with the quilt, that's 16.5oz (would that be warm enough?) for the Arc AT+parka, whereas now I carry a Nunatak Arc Specialist at 16.5 oz + 8.5 oz for the quilt.
The problem is, at 45 degrees and up, I'd usually carry at most a 5oz vest or just my wind/rain shell, but those wouldn't be warm enough combined with the quilt to sleep in.
I'm might be trying to overspecialize here, which is what I'm trying to figure out.
I suppose in need to experiment at home, sleepin at night with my bedding just up to my chest and see how I handle 60 degree temps in lighter tops.