Just updating on this whole thing.
I saw a good deal on a size small Uniqlo down parka on the Gear Swap, so I went for it. This piece, by contrast to the last, is sized perfectly for me, and I'm totally thrilled with it. All previous accolades apply. Soft, light, puffy, seemingly decent construction, simple detailing. This piece is going to be getting a lot of use from me.
Will be an ideal packable commuting layer for me, when I'm off the bike and waiting at the Austin train station(s). As for backpacking, this is a worthy successor to my Stoic Hadron -- it's just as warm, but includes a full zip as opposed to 1/4 buttons, and has more normal / manageable cuffs.
I believe my size small clocks in at about 8oz. In the neighborhood of the 9oz MB UL Down Inner Parka, but probably a tad less down fill.
I like this piece enough that I might consider upgrading to Uniqlo's "coat" which seems similarly constructed but offers better butt coverage. If it goes on sale, that is. Or maybe not. At $90 it's already a steal.