If the down jacket were full length, that might work... I don't know that down gear is truly additive as each layer compresses the one under somewhat. I really like the look of Nunatuk gear, especially the Backcountry Blanket, but my budget is more DIY and JRB sized. And the JRB quilts are multi use, too.
I use two JRB Hudson River quilts - they work as top quilt or bottom quilt, but they are full length. If I were counting ounces I would get a Yeti and take one of the JRBs. I'm still considering throwing together a DIY Yeti style with some synthetic insulation I have.