I got this crazy idea. Sleeping bags are more effective at keeping you warm if they fit properly - loose enough not to collapse the loft but tight enough not to have unnecessary air space. So why not just do away with a sleeping bag altogether and use a coat and ski pants instead? They're close fitting, warm, comfortable, and potentially even lighter and less expensive. For nights when it's a little colder, you could use a vapor barrier layer over your base layer so as not to wet out your insulation. If you sweat too much and get your base layer wet, the VBL would still hold the heat in, and I bet this setup with an extra dry base layer (for day) would still be lighter and less bulky than carrying warm clothes for camp and a sleeping bag. Of course, I know it wouldn't work for extreme conditions, probably no colder than 20F at the lowest (just guessing), but for moderate to warm climates, do you think this would work as a comfortable three season setup?

Or, if you want to allow your legs to share warmth (like how mittens are warmer than gloves), you could use a half length sleeping bag. Just thinking out loud here.