Well since you asked, here is what I'm thinking;
For one I still can't see how you "feel" the rearend moving around under the car. Not saying its not possible but saying that you are an ex race car driver does not make it so(sorry for sounding so blunt). Second it seems that you are looking for something "better" than a leafspring setup and trying to add on more to it to make it "high end"
Have you even thought about the other suggestions? It appears that you are more stuck on buying some new fandangled suspension ad-on that is not even needed just for cool bonus points. Before I went and dropped some coin on something like a watts link that in the end after paying someone to install it is gonna cost you probably $1000 I would be looking at more simple things to actually fix the problem, not just throw fancy parts at it that aren't needed.
There are many racers and people driving leaf spring sprung cars w/o bandaids like panhards and watts links with much success. THere have been threads here and PT.com where suspension engineers state that adding somethingi like this on to a leafspring suspension is NOT a good idea, yes it is doable but it needs to be well thought out. I don't see how the driving your doing is so much different that warrants something like this?
It seemed to me you just wanted to bolt something new on the rear suspension to "seem" better to match the frontend of the car. Like maybe you came up with this whole idea of the rearend shfiting out of thin air to make a good excuse to bolt on a watts link because they look racey. a set of del-alum rear bushings are under $200, and removing the rubber pads from the rearend housing is free. and both of these could easily fix the so-called rearend shifting problem that you think you have.
Its your money and your car(really nice car BTW) and of course do what makes you happy, but if you really just want to fix the problem than I think you should start out with the simple/basic stuff, before you go through your money away on "racey" parts to compete your idea of a "high-end" rear suspension.
do a search on PT and here for info on using these systems on leaf spring cars. I know David Pozzi and Mean69 have a few posts with some good techinical data for you. Sorry I can't provide tech data on why its not a good idea, but I'm no guru
but maybe you can make it work? Not saying that it can't be done, just that you would want to do some research before you shell out the moey.
EDIT:
I just read his tech on his site half-azzed and it seems he thinks that leafspring suspensions can benefit(duh, he's selling them) so maybe he's on to something, but his site does look like one huge ad I'd be a little skeptical. Its been explored previsouly like I stated, but maybe we can get some suspension tech guys to examine what Fay's is claiming and comparing it agains their own opinions. I still stand that you should first attempt to fix the problem with more simple solutions, but I also think that these systems could be explored a little more. I don't have the knowledge to really say whether it will work or not, but only that others who do, have said not to run them