If the $500 price tag doesn't scare you, the Western Mountaineering Terralite might be one to consider. It's a "semi-rectangular" mummy bag, 25 degree, and pretty roomy (inside girth 65"), 1lb. 13oz. in the medium size. Expensive, but US-made, and WM's customer service is excellent in my experience.

I saw one of the Nemo bags at REI the other day, rated at 15 degrees, but obviously only for very warm sleepers, as it had only about 4" of overall loft. I notice on their web site that the "Riff" bag (not sure if that's what I saw) is rated at 15 degrees, but it lists "ISO lower limit" at 16 degrees. Seems goofy.

You can save some weight by using a 3/4-length pad, especially if you carry a sit pad anyway.

If a CCF pad is an option, look at the Nemo Switchback. Similar to the Z-rest, but somewhat thicker and the same packed size. I bought one of these last fall, but haven't used it yet, although kitchen-floor tests seem promising.