I got a Kelty Lightyear Down 20º bag that I like a lot. I think it might work pretty good for you too, based on your description. It weighs under 3 lbs. I think the new version is lighter than mine and uses higher fill weight down (750 I think).

This past season I learned to put the sleeping bag loosely in a heavy duty trash bag, then put that inside my pack, then put my gear in the pack and let the gear compress the sleeping bag inside the pack. This compresses the sleeping bag less than the compression sack it came with, and gives you more room in the pack because there's less unfilled air space inside it. You might want to try that with the sleeping bag you have.