I'm aware that a kit is being developed to use C5 rear calipers and the bigger rotors from either C5 (12" x 1") or C6 Z51 (13" x 1"). These variants will, unfortunately, require larger than 15" wheels.
Maybe if enough interest is shown, a version which uses the SS rear rotors--to fit 15" wheels--would come to pass, but I think between Gerry's effort, and the exchange of ideas with both KORE3 & Gerry, the consensus is that its better to use the C5/C6 rotors, thus the issue of larger wheels.
Using the thinner and smaller SS rotors really does not make an "improved" brake setup, considering the wagon's greater rearward weight bias. If you're gonna spend the money, make the change that gets the most "bang for the buck".