Which gas to use always makes for a spirited discussion. I'll just report that I've used nothing but 87 octane in my 3.3 in my car's 5,000 miles. Per the car's computer, I consistently get 21 mpg in mixed driving (and that's with very little freeway driving - probably about 10% of my miles). Don't know yet about highway mileage, since I haven't had a road trip in it yet.
Before we debate whether the car's computer is accurate, just note that my previous 2.0T Stinger got identical mileage on its computer! So I'm comparing apples and apples. I was ready to accept worse mileage in my GT2, but to my surprise, it's the same as I got in the 4 cylinder car. And I have plenty of power on tap. So I'll just keep the extra money in my pocket ... over $100 saved in just 5,000 miles.