If it came to keeping warm in single digit temps, the weight of the gear would come second. I would be more that willing to carry a few more pounds if I knew it was going to keep me warm so that I could get a good night's sleep and wake up refreshed and ready to hike. I good nights sleep will probably be golden on such a long hike.
To answer your question Michelle, I'm pretty sure my listed setup would be fine down to 10 degrees. I don't really know how my top cover is going to perform, but I am hoping for at least 8-10 extra degrees of protection. Here's what I listed:
Hennessy Hammock ULBA
Gear Guide 9x9
UL JRB Nest
UL JRB No Sniveller (also as underquilt)*
DIY Top Cover
Western Mountaineering Apache SMF 15 degree sleeping bag
WalMart Blue CCF Pad*
1/8" Ultralight CCF pad (3/4 length)*
* denotes items that would be used for temps lower that 20 degrees
I will probably wear some mid-weight poly/wool thermals, fleece socks and a fleece cap, adding extra thermals or a fleece pullover if needed. I can't sleep well with tons of clothes on; it bothers me. I will probably start with the bare minimum and add items as needed.
I'm a pretty warm sleeper but if I were expecting single digit temps I would add clothing first and I would also add a fleece sleeping bag inside my WM bag. Extra gear for single digit nights could be included in your bounce box if you didn't want to carry it all the time.