Thanks for your responses! I'm glad this piqued some interest, & hope it helps out more people.
A few comments about the discussion... Field testing a sleeping bag for relative/comparable warmth is an inherently subjective undertaking, so I eliminated as many variables as possible. While realizing that some people on BPL use low-rated R-value sleeping pads, for example, I used the same pads I always do. If I had used a lower-rated pad, I wouldn't have had a good comparative baseline. Also note that there have been some great threads correlating sleeping pad R-value to temperature & sleeping bag rating.
In this one:
Ken Strayer noted that EN standards for testing at 20*F use an R-5 value pad; by comparison, R-2 for 50*F and R-3 for 40*F. I can't find the post right now, but I seem to recall these numbers being supported in another thread by Mr. Nisley. The point, of course, is that to determine if a manufacturer's rating is accurate, it wouldn't be appropriate to use a lower R-value pad than they use for testing.
The two pads I used for testing were the Thermarest Women's Trail Pro (R-5.1) & Exped DownMat 9 (R-8). The R-8 pad, sufficient for subzero *F temperatures, was the pad I used on the cold-sleeping night in the teens. It was clear that all variables were unchanged except for the daytime/nighttime temperature swing.
For those interested in such things, Casey's calculation on loft can be a good starting point... However, for the numbers to be accurate that calculation assumes that the amount of down is going into the same-cut sleeping bag shell & that the shell fabric is the same weight. Given that the two bags compared have a difference of 3" in shoulder girth, and have different-weight shells, the actual "loft ratio," if we were to call it that, would be more in favor of the Western bag.
All that said, my goal was to emphasize real-world, practical findings... more so "Does it work as advertised" than "What do the specs say," ya know? And hey, the good news is that it DOES work pretty much as advertised. Now if we could just find a $150, sub 3-pound, double wall tent ;)