Yes, on my autobahn there is no limit (my Mini tops out at 150mph (gps checked) but I drive over 500 miles a week and I try anything I can to keep the costs down. As for what cars run better with each octane type, every car is different but rule of thumb is run the lowest octane your car can run without pinging or detonation.
With 4k miles you should have done the break in oil change. Metal particles= bad. I know it isn't covered in your free maintenance but for the break in oil change bite the bullet and pony up the cash yourself!