“travel light, freeze at night” - random baby ranger
Take you current sleeping bag and puffy jacket and sleep with your bag around your waist next trip. You will figure out pretty quick if it is for you.
those things used to be very popular with the alpine crowd before ultralight packing was a thing. Adventure racers, fast packers and the hard core oz counters love them. Honestly, the difference in weight is probably a pound And your gonna need a pretty heavy jacket to make up for the lack of coverage. If I were gonna try it, I might think about a lighter weight, higher rated conventional bag (40-50*) and wearing all my layers. Just make sure you have a bail out plan or can build a fire if it doesn’t work out. Lots of ways I think think of to lighten a load that much without compromising my sleep comfort.