I always love to see more real-world validation of the influence of binding mount position. As you noted, +1.5cm is really where you're getting into the right ballpark on a lot of Head skis (Stockli too).
Interestingly enough (and for those that think I always wants a forward mount), I was skiing with ToddW this past Sunday and noted that my Atomic Savages were feeling just a bit too much "on/off" through turn initiation. They were almost too grabby in the snow conditions. I backed up the mount position on the Railflex to -1.5 and the skis completely calmed down. They were much smoother through transition and I felt like I could stay forward on the skis without the tips fighting me. Based on where the rails were mounted on the Savage, the -1.5 position actually put me right back on the factory mark for that ski. So here's a ski where I actually preferred the factory mark over my initially calculated position.
This all goes to show that in the end the best advice is to use a binding that permits easy experimentation with your mount position. You'll never really know just how much performance you can get out of your skis if you never try any other positions on them. You just might not know what you're missing.