Actually, though I'm not an expert about all three road RD brands (Campy, Sram, Shimano), it seems there is no mechanical difference between the RDs. The differences are in the shifters and cassette. How much pull a shifter moves the derailleur over the cassette and how the cogs are spaced is the key to precise shifting. The parallelogram movement shouldn't make any difference in it's function. It's just a mechanism for moving the chain.
That's why I posted the question can a SRAM Red RD be used with an all Campy drive train. I guess I'll find out when I get the derailleur and test it since no one, yet, has come forward to verify this. Kind of an interesting experiment just like Zinn's article. Besides, if is does work, that means more grams save using Red over Record for the RD.
You're a bit confused. If there was no mechanical difference in the RDs, then SRAM and DA shifters would pull the same amount of cable to make the RD move the same amount, since their cog spacing is the same. The parallelograms are not the same, so the shifters pull different amounts of cable to perform exactly the same job. If the RD's were all the same then you could just put a Campy RD on a SRAM or DA bike. I can guarantee you that won't work.
The folks a J-tek have all this figured out and that's why they offer so many models of shiftmates.
Although A J-tek can correct the total cable travel, I don't believe that they can make a SRAM shifter work exactly like a Shimano or Campy models, since those don't pull equal amounts of cable on every shift. About the best that can be done is to produce an average pull that results in the correct total cable travel and RD movement.