Hi cobba,
You are partly correct. Thanks for some of the piston sizes, I already had some of them.
The TRP that I have (2011 version) has 21 & 19mm pistons driven by 10 & 9.5mm master cylinders, where the 2012 version has 21mm pistons F&R with dual 10mm master cylinders.
The Avid X0 trail calipers were the closest in terms of matching the ratio correctly. The Saint calipers would have given a much bigger piston area and hence a mushier lever.
I had a look at a lot of calipers - trust me
I was after a combination of better pad area and matching piston sizes.
I had forgotten about the doppelmoppel, but when you look at the cost of $380 + shipping, it's as expensive as the Plan B option (to be revealed later !!!) that I took and that option includes calipers as well. I may still get the custom seals (will check on Monday), but enjoying playing around.
In the end, I'm wasting money, but enjoying it and the challenge.
On a separate note, the new pads that I put in are good, and so far on a flattish ride, about 10 - 20% better than the stock TRP Resin pads