Put.com published in a recent article about twin turbo V6 engines that Ford confirmed that the Ecoboost does in fact gain more power with premium fuel in stock form. The Ecoboost gains another 20hp and 10lb+ft of torque on premium fuel.
"We should note Ford tells us that its power outputs improve slightly, measuring 385 horsepower and 430 pounds-feet of torque when using premium fuel, instead of the regular fuel the owner's manual recommends."
Seventh paragraph, last sentence.
I guess that finally ends that argument , but of course the whole "better fuel economy with premium" will still be debated.