Any data to support that?

I can't imagine just because it's a Japanese plug means it doesn't work in a German car. It also doesn't explain why the car ran fine for 12k miles since I changed them. FWIW, I had Bosch resistor plugs in the car for a while too, and it ran no different by feel, mileage, smoothness, and power. But, I did get non-resistor ones just to be sure.