I haven't done any camping at temps that low - hammock or otherwise. I would guess you'll get the most bang for your buck in an upgraded underquilt, a tarp you can close the doors on (think a full, floating tent... something like the Warbonnet Superfly or a DutchWare winter tarp), and a really nice / warm topquilt.
As always, I'd recommend checking out Shug's YouTube. Here's a list of his videos related to winter camping, and I am sure there are some gear recommendations in there.