My 20* sleeping bag (WM Ultralite size short) weighs only 25 oz. I'm not to sure why I'd want to double the weight to add maybe 10*.

Why not add a few ounces more down instead? Or wear your insulating clothing inside the bag? Or, ir it's below freezing, try a vapor barrier?
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey