I love using a down comforter type system. For the past several years I havent slept inside a sleeping bag but instead I unzip it fully and use it as a comforter. The key to warmth and comfort is the pad which is largely a personal choice. I have used my -5 degree bag like this in all temps and have never been too cold or too hot. Down seems to work better than synth. because down tends to drape over the body better.
I used this system on a 9 day hike in the Sierras last year and had temps between 50 at night near Bishop and 20 degrees at 12,000.
If yopu havent tried this system I would highly recomend giving it a shot.