Sleeper cranking out 100 watts.
Temperature difference of 50degK, or 90 degF.
A surface area of 2 square meters.

So you need a Rsi value of 1, or 5.75 in english units.
A clo value of about 6, more or less.

Shouldn't you be able to achieve R 5.75 with 1" of loft, maybe 1.5"?
If say half the heat is lost by other means, like breathing or leakage from aroud the head and neck area, then perhaps 2" or 3". But aren't those losses reduced to 20% or so with a good design?

So how effective is 600fill or 900fill down, per inch of loft?
How much does it depend on the shell material?
How do bivies and ground pads tie in?
How about wool liners?