I've tried it the other way. Where I leave my puffys at home and get would get out my sleeping bag when sitting around and needing extra warmth (glassing, at camp in the evening, etc.). That actually works pretty well as long as you don't need your puffy when moving/hiking.
I'm not sure it would work well to just sleep in your puffys. Seems like you might lose a lot of heet at your waist. What about your feet? Seem like they could get pretty cold at 20 degrees. And leaving them in my boots 24/7 sounds pretty miserable. I'm sure you would survive. But, I would imagine it would be a pretty awful nights sleep. Maybe it just that I'm getting older, but I really value quality sleep when I'm in the backcountry. IMO, I would find somewhere else to cut weight.
Either way, I love a good experiment. I tried leaving the sleeping pad at home for a while. I successfully saved weight and could hardly get any sleep.
A woobie weighs almost 2# if I remember correctly. If you’re bringing that, you might as well bring a quilt instead. A 2# quilt could be rated down to 15-20 degrees for a high end model and maybe 30-40 for something with synthetic or lower rated down.
It is less efficient, really like glove vs mitten. Plus, I don't imagine the comfort would be great unless you were using synthetic which can be more breathable vs down. Where you may get some benefit is like a hooded belay jacket and half bag IMO .. but still overall comfort probably would be low. FWIW, most, puffy jackets, on their own, are good to about 40 degrees in a non moving environment. Moving , sure you can take them -0. Sitting and glassing, while not moving is still not the same as being asleep.
I called up Jan at Nunatak with the idea and he was able to make it for me. Putting a zipper on the bottom of the jacket allows different design to be zipped on, Plus you can have the jacket made to your specs.
Yeah, Nunatak makes great stuff, just like that Raku sleeved sleeping bag. But, their wait times right now mean you won't see anything from them for about 3 months or so and they aren't making their full range of products due to staffing shortages and supplier issues. They are closed right now from July 15th-Aug 17th and aren't taking any new orders. You have to plan ahead when you order from any of the small cottage manufacturers.