Maybe a fuelish question?

Depends on the year. In the last 10 years or so the computer adjusts out the "ping" so by running premium it would not retard the timing so much and could give you more power and could result in better mileage. However the computer doesn't do that instantly, it has to "learn" so it could take a couple of tanks before you see the difference in the octane level to performance. On long trips I put in premium, normally I run mid-grade.

Also before you make to drastic of a decision understand your compression ratio. If your vehicle has a 8:1 ration premium would not do anything. If it is 10:1 you better be running premium all the time, your manual would recommend it at the high.

Diesel only... :)

Theoretically you would. I don't know if it would be worth the price premium tho. You have to do some calculations.