I am not too sure if the underquilt is such a good idea. One of the problems I have with all hammocks is the extreme amount of underlayers needed. Coupled with body compression laying on top, and they often start bottoming out at around 20F-30F, regarding weight/per insulation. Often a more rigid foam type insulation is wanted, like a closed cell foam (CCF) pad...often a couple or three layers. It is hard to find a better insulator than snow, though, it is only good to about 28F-30F, then it starts getting wet with compression. Generally, thick pads are used as much to keep the melt managable, and away from the bag, than to supply real warmth. Though it works both ways...
Anyway, 550 fill is likely a good choice for stuffing between two layers of synthetic fill. It will resist compression some, but also trap small pockets of air in it, leading to some insulating effect, even laying on it. Good high lofting down will likely perform no better, because of the compression. The downside? Well, it will likely perform no better than an equivalent weight of the synthetic fills...it will just last longer if properly laundered. Without some thurough analysis, it is hard to say. So, take this as a best guess by a semi-literate.
Fact: 550FP is about equivalent to many synthetics in insulating effects when new.
Query?: 550FP will compress a little more, but might be damaged more do to the preponderance of stemmed breast feathers with down-like barbules, instead of down plumes (known, but hard to tell how it will compare to synthetics.)