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Reading Bill Taylor's graph regarding his proportioning valve test results, I disagree with your assessment. The Pantera "proportioning valve" does indeed have a knee in the curve, at 650 psi. Below 650 psi the input equals the output, it is linear; above 650 psi the pressure is reduced by about 33%. It would be much more obvious if the horizontal scale were expanded.
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Sure ventillated rear disks are preferred, optimal, to be recommended. I would want them for my Pantera if it didn't already have them. But ... I've seen many US manufactured cars get away with solid rear disks, ventillated only in the front. This is why I've repeatedly mentioned only ventillated front rotors. The fronts are more essential, and my comments to Adams were originally the bare necessities.
Some people don't want to spend any more money than necessary, others want to keep their cars as original as possible, changing only the essentials. My comments have been geared for such owners.
But its a moot point now, because in the future my advice regarding Pantera brakes shall be:
(1) Ventillated front rotors
(2) EBC, Porterfield or Raybestos front pads
(3) SACC Restorations rear brake kit
(4) Remove the proportioning valve and mail it to Mike Drew, he loves them
I feel accurate calling it a proportioning valve now!
-G