Premium except in the hottest of summer here in 'Vegas when air density is low, as in higher altitudes. Then I use mid grade gas.
Premium gas takes advantage of the higher 11:1 compression ration of my 1.8 L. engine. It gives me more power! (according to my dynamometer)
But regular gas dyno readings were taken when the car was delivered and the engine was not yet broken in. I'll do more testing at 8,000 miles.