As my younger granddaughter says, "Me, too!"

I do something similar. The Megalite is a 30-degree bag; usually, if it's going to be colder than that, I simply choose to stay home. (I'm not as much of a winter camper as I used to be.) I also carry a down sweater and long johns, which serve as my backup system if it turns out to be a little cooler than expected. It saves weight, and gives me a nice safety margin.

I'd also mention that WM bags are conservatively rated, by about 5 degrees. Marmot bags are pretty much dead on.