I'm glad you did that experiment. An experiment is worth many many calculations.
With a little additional information, your home test should be quite convincing for ballpark numbers -- certainly enough to resolve between the factor of 4 or 5 differences between claims. Do you know the approximate temperature of the hot pad? If the hot pad control setting fixes the power output and the geometry in each case was roughly the same (or, if you lay the garments flat on the pad, and you assume the power output of the pad is evenly distributed over area), you can obtain relative R values for each garment by calculating the temperature gradient.
Is it correct to read from your graph that the temperature on top of the fugu piece was 70 deg/.93 = 75 deg F, and on top of the WT vest or jacket 70/.81=86 deg F? Or did you use deg C, or K for temperature units?
If the heat pad temperature is around 110 F, you have temperature gradients of 35 degrees and 24 degrees, respectively, for down and .6" PL -- so the 1.5" down is about 1.5 times as insulating as the .6" PL (2.2 clo). That is about 3.3 clo, closer to Ayce's experience. If the hot pad temperature is lower, say 90 deg F, then you have temperature gradients of 15 and 4, and the down is about 8.25 clo, closer to your calculation.
Lots of assumptions here, but it seems like we should be able to get within a factor of two.
I'd like to do this test myself -- but I don't own any down garments!