FWIW, I have been intrigued by the notion of a quilt but have found a bag more versatile at only a small if any weight penalty. I use my down bag as a quilt sticking out and arm or leg or two to regulate when it is fairly warm (above 45 or so), completely over all but my head when it is chilly (down to maybe the upper 30's), or zipped up when it is really cold adding some clothing and pulling the hood tight as needed. Extra socks are a must when it is really cold. Also using a bivy conserves heat a good bit.

Given that my bag is only 17 ounces there really isn't enough weight to be saved to tempt even a gram counter like me to give it up for a quilt.

I should emphasize that this is only what I find works for me and it may or may not work for others. Also I put out heat like a furnace so my beloved Mountain Hardware Phantom 45 also may be insufficient for most folks in the conditions mentioned.