Paul, during winter I use the following to sleep:
Arc-alpinist, with 2 oz overfill
Nunatak Bacalava
Bibler winter Bivy – to seal out drafts and add few degrees.
Golite Tarp
Recently switched to Stephensen DAM pad. Very warm/comfy
Mid to exp weight Wickers base layer bottoms
100 weight fleece base top
sometimes 200 weight fleece also
Nunuatak Down Booties – very warm
I have been comfortable in the teens, but it never got into single digits where I go.
I just bought Flight Jacket and have Montbell down Pants on christmas list to get pack weight down.
However, my concern with sleeping with down clothing on especially with Bivy is your sweat wetting out down clothes thereby making them less insulative when needed around camp. If you are in humid weather, of which I am, I think this could be problem in morning before it has time to dry out.
Has anyone else had a probnlem with body produced condensation in down clothes while sleeping?
Chuck