My down vest measures a little over 1 inch in thickness (inside to outside) and it is probably good to 30F. Has sewn through construction.
I have another one, sewn through, but has a fabric lining so the effect of the sewn through seams is diminished. I haven't worn this one in several years, and don't recall how warm it was. Also a little over an inch in thickness.
It is new, and I haven't worn it yet, so I can't say.