It's 99% advertising hype. The oil industry pushes premium gas since the profit margin is higher. Car manufacturers want to lead buyers to believe that their car is so finely tuned that only the best gas will do. As far as I've ever seen there has never been a formal study to indicate that a change in octane will change your mileage. The percentage change that most drivers claim to get could easily have been achieved by any number of different driving conditions between fillups.
All Porsche engines are designed for premium, too, but it's not available everywhere. "Our cars must be able to drive all over the world, and so we are able to run on regular," says Jakob Neusser, director of powertrain development at Porsche's research and development center in Weissach, Germany. "You don't have to feel that a mechanical problem or anything else will happen" using regular gas, even in the highest-performance, regular-production Porsches."