You may want to consider an 'overbag', such as a Big Agnes. Significant room, and it works with an inflatable pad, so they are roomy and comfortable.
Horse Thief is rated to 35F, that seems to be a pretty honest rating to me, the Zirkel at 20 F.
One of the nice things about an overbag is you can also get a lighter less expensive bag, put it inside (hence the 'overbag" moniker), and take what warmth you need.
I toss and turn a lot, so it's very nice to have a lot of extra space, and usually it's at a minor weight penalty.
I think there were some listed on this site, and if not, I know someone selling a Horse Thief.
Does that help?