For a number of years now, I have been using the Feathered Friends Vireo (half bag) with the FF Volant (down jacket & hood).
http://www.featheredfriends.com/Picasso/Bags/Specialty.html

I have done just fine with this combo in below freezing temps.

If I had a full length sleeping bag, my down jacket would have just added 1.5lbs to my pack weight. Then used as a pillow. Who carries a 1.5lb pillow?