On the eve of investing $300 to have a shop take some of the lumbar support out of my new seat, I dug in with both hands (I guess the $$$ was a motivation) and found the cure.
Some people have asked me to post the results if I find a fix, so here we go:
I removed the seat back (I found a great tip for doing this on the 7 series board). My inability to do this was holding me back from a self-repair.
I discovered (I had already cheated and had obtained a part list graphic
showing the key components) that the seat back assembly essentially consists of
one molded piece of foam resting against a series of horizontal and vertical
wires attached to springs that ultimately attach to the seat frame (very similar
to an old metal bed frame). In addition to this mesh of thin wires, there was a
very thick wire in the lumbar region of the seat back with the following
_ _ _
_| |_| |_| |_
It ran from the left to the right seat frame, similar to the other support wires. This had give to it, much like a spring, however, it was very stiff. For those that think the lumbar region is too stiff, THIS IS THE CULPRIT!
I used a piece of 12 Ga. electrical wire to pull the center (by pulling the center, the spring maintains it's arc) of this spring all the way back to plane that represents the edge of the seat back. I tied this wire to a toilet plunger (don't laugh it saved $300). Of course, I will device a more permanent rigging once I confirm that I like the fix and there are no down-sides.
Viola' no more pain. I can focus purely on what a great car the 540 is.
The beauty of the fix is that it is not permanent (simply remove the plunger, LOL) and you can variably adjust how far you retract the spring to fit your particular shape / discomfort.
Good luck to all. I would be happy to post additional details if someone needs them.