I did a test with this on another car of mine (accord). They are made to run with reg, but I filled up with prem every other for a month or so. I seemed to get better milage with the prem, and slightly better drivability. Perhaps this is more apparent with older cars as they start to develop carbon deposits etc in the intake and combustion chamber. In any case, the slightly better milage didn't justify the higher cost of the premium.
I always use premium in the SVT. It would be different if you could manually advance/retard timing in cases where lower grade is only available, but that is not possible with the duratec. I wouldn't put anything else besides premium in it. If anything you will have a cleaner and more efficient engine even after years of driving.
