I have a 0F Marmot Couloir. Just the right fit for cold weather, when you may want some extra room to move around in the bag to take socks off or put them on, etc. Still really light and has water repellant material on top so frozen breath that settles on top at night won't melt and get your down all wet and soggy.
REI still shows the pack on this page, long discontinued:https://www.rei.com/product/731796/marmot-couloir-sleeping-bag
Got it on Steep and Cheap a few years ago at 60% off (less than $250) and it will probably be the last cold weather Sierra bag I'll ever need. Ripped a hole into it with my crampons, but patched it and it's all good again. Now it has character.
Today, you'd have to get their "Lithium" bag to get a similar model. $500 unless on sale.
I also have a -20F Mountain Hardwear "Wraith" which is amazingly warm, but also heavy and hard to pack. Too warm for Whitney in April, but I may bring it if I ever do my winter Muir Trail, as it is a warm cave all in itself.