OK, I talked to my tranny guy, who also happens to work at a Ford dealership. The servo letters do not run in any sort of order. Yes, the 'R' servo is supposedly the best one, but those usually go for big bucks. My guy just went to a boneyard and took a look at thier tranny pile. He popped off the servo covers of all the C-6s he could find and took a look at them, taking the biggest for his race trans. He wasn't paying any attention to the relative sizes of the servo/pistons so I can't tell you what order they go in.
Not only do the servo/pistons vary, but there is a pivot lever inside of the trans that also makes a difference. On a C-4 the servo piston acts directly on the band, but the C-6 uses the lever. The servo piston acts on the middle of the lever, with one end of the lever pivoting on the case (fixed) and the other end pushes on the band. There must be a problem with those levers in racing use since they are known to break often if the pressure in the trans (and to the servo) is run too high. Someone makes a billet aluminum lever that is stronger than any of the factory pieces.
Hope this helps you somewhat.