This pad was an experiment. It weighs 15.85 ounces, it is 64" inches in length (for my 62" long girlfriend), and it contains seven ounces of 800fp down. When inflated, it is 3.75" thick over the middle five baffles and the outer two are 4.25" thick to prevent rolling off.
It is something between a "balloon bed" (http://www.balloonbed.com/) and a Kookabay pad. Like a balloon bed, it has a thin, elastic bladder inside a fabric envelope. Like a Kookabay pad, it is meant to be durable and the internal volume is contiguous (it has a single inflation valve).
To make the inner balloon I used a hobby iron to make seven tubes from a tough, stretchy heat-sealable urethane vacuum bagging film called Stretchlon (http://www.airtechonline.com/whatsnew/index2.htm). The film weighs about 1.2 oz/square yard. I then joined the tubes at one end and made holes for air to pass between them. I then nicked each tube, added one ounce of 800 fp down to each, and resealed them. The balloon is bigger than the fabric envelope, so the film is not being stretched when it is inflated. The fabric bears the tensile load; the film is just an internal air barrier. The valve is a 9mm kitesurfing kite inflation valve.
The fabric envelope is "coyote brown" Momentum 90 from thruhiker. The baffle walls are made from 0.93 oz/yard spinnaker and are 3 inches high. There is a hole for the valve to protrude, and openings (with snap closures) at each end for easy removal of the inner balloon. The balloon is loaded with down for shoulder season alpine and winter use, but it could be replaced in summer with an unfilled inner balloon.
I think, with a lighter fabric like SevenD, a 72" long, 3-4" thick pad with 6-8 ounces of down could come under 16 ounces using this design. I don't have any guess about the R-value, but I don't know of any other pad in this weight range that uses this amount of down. I'm pleased with the result and I expect my girlfriend to be much warmer than me (on my Synmat UL7) this winter.
Any ideas or feedback are welcome.