Roy,
Thanks for the long response. Here are a couple links that show interesting data on the above topics:
www.t-linespeakers.org/projects/tlB/radresponse.html
This has impedance data which shows a remarkable impedance flattening at the 1/4 lambda frequency at quite reasonable stuffing densities, in addition to a dramatic reduction in the driver resonance peak itself.
www.t-linespeakers.org/projects/martin/focal/test_line.html
This also shows a dramatic drop in the 1/4 lambda resonance at normal stuffing levels (using Dacron), and also has several other interesting results. One is that the reduction in speed of sound is far less than Bradbury etc's data on wool and fiberglass. You are likely correct that the microscopic fiber characteristics have a major role in this. Also, note that at the higher frequency peaks, the experimental data show near-perfect correspondence with the theoretical numbers, suggesting that there is effectively NO air-mass coupling to the cone at these frequencies. This one plot is what convinced me that there is indeed a strongly frequency-dependent air-mass coupling.
I will still take issue with your (implied) statement that added mass is not a problem. I understand the games that can be played with mass and compliance, but only at the expense of cabinet size and/or efficiency. I also understand that one can say that "you can always make the magnet bigger." But therein lies the real-world problem: you would like to keep the efficiency as high as possible (within reasonable limits), and the cabinet at a reasonable size, while being limited by the reality of the relatively weak magnetic fields achievable with fixed magnets. So added mass does not come without penalty. In addition, my passion for a long time now has been for 2-way systems, so my perspective tends to be skewed by that reality without my realizing the need to state it, and I should have prefaced my comments with it.
Again, thanks for the extraordinary effort you have put into this thread. It has been very enjoyable.
