Another vote for Western Mountaineering. I've had a couple of their bags, and found them to be about 5 degrees conservative. So, you might look at the Western Mountaineering Megalite, which is a 30-degree bag - but I've used it with longjohns down to 25 degrees, and with a down sweater to 20 degrees. Add light down pants, and you're good to 15. The bag weighs just over a pound and a half; the clothing is what I'd be taking regardless of bag choice.