I use my down jacket + added down pants (that weigh ounces compared to a bigger bag) Feathered Friends down booties (without the hardshell, they weigh a few ounces)Over Therma silks, and it turns my +20 bag into a 0 degree bag comfortably (sleeping in a silk cap adds to the warmth)

The body betrays and the weather conspires, hopefully, not on the same day.