Yeah, driving in the alps does change characteristics of the car, but to worse. I noticed the higher I drove (altitute), the worse the power, torque, everything. So loosing 10% on fuel economy is a good thing

I took the X5 to Italy in July through the alps and experienced the same thing (loss in power, etc) using 98 octane all the way. So I assume that in the opel I had higher octane gave me better fuel economy.