Excellent read Bill and I applaud your efforts to find the perfect set of spring for the MagneRide on our cars. I finally installed the TTRS springs and shocks (MagneRide of course) on my 3.2 Coupe this weekend to replace the stock MagneRide. This gave me about a 0.5" lower ride height but I don't know the change in spring rate.
One piece of information on your post I have a question about is why a stiffer rear rate will help keep hot tire psi lower? I am in the process of upgrading the linear 400lb front and progressive rear springs on my Z4MC KW Clubsports to 517lb linear ones all around this winter and was wondering what to expect before I try it on a track? My hope is to get the suspension to a point where I can fine tune with tire pressures on track.