I was tipped off about this a couple of years and was amazed that it worked so well. The Z didn't need it so I tried it out on our Aygo, which did, and it worked a treat.
Assuming that both discs and the pad set have been installed for the same length of time, possibly some contamination has happed by way of some form of grit, therefore scoring the disc.
Brave man, I'm very impressed that you went through with it.
We popped into Apex last year, as we do during most visits to Nurburg. Misha and the crew are always good to chat to and very welcoming.