Haven't tried using premium in my new SEL hatchback but I do know it made a difference in the 2000 Saab 9-5 Aero Wagon that my SEL hatchback replaced. In the Saab, it was shown in third-party testing that running premium gave 11% better gas mileage and horsepower over regular. That I can agree with. I saw my gas mileage drop approximately 2-3 mpg as an average using regular vs. premium. And in the Saab, I would experience engine knock under acceleration with regular gas and none with premium gas.
I would be curious to if anyone who is currently using premium in their Focus what their increase in gas mileage is and would it justify spending the extra money for it.