Ray and his wife have done cold weather trips all over the planet using his synthetic quilts instead of down bags. Read page 18 on his quilt section of his website.
http://www.rayjardine.com/ray-way/Quilt-Kit/index.htm

The way sleeping bags or quilts work is that they trap the air in the bag that your body warms up and insulate the warmed air from the ambient air outside the bag. Regardless of how the bag or quilt is constructed, that is what is doing.

Some bags are more efficient at doing this because of their shape (mummy v. square cut) and the amount and type of insulation (down v. synthetic) in the bag. Ray claims his quilt is as efficient, if not more so, than a down bag. I've never used one, so have no idea if that is the case. His many trips seem to bear this out. Keep in mind his quilt looks like a sleeping bag without the bottom on it, which he says you don't need since you are compressing the bottom layer anyway. Not sure what he uses for a pad, but I would assume he talks about that on his website.

One reason bags have hoods is to slow down the heat loss from your head warm. Your head is a big heat exchanger and in cold weather, you have to keep it insulated. You can buy a separate hood for really cold weather from this guy-

Innovations by Finbar

There may be other makers of hoods out there as well. Ray has a simple version of a hood on his website.

There are bags that are more generously cut than others for people who want more room. I have a long bag even though I am not real tall. I like the extra foot room, but that means I have to warm up some extra air.

I still like the idea of getting inside a bag, but based on Ray's many years of adventuring, it seems pretty obvious that a quilt works just as well or better. Ray uses synthetic because of his bad experiences with down as he recounts on his website. Some top of the line down bags use Pertex, eVent or some Goretex variation to keep the insulation dry.