To a first approximation, engine power is about nothing more than how much gas per unit time you can burn. Sure, there's an efficiency factor, but that's pretty close on most cars of similar type and vintage.
The nice thing about the ST is that it can get both good mileage and good power. That doesn't mean it's going to do both at the same time. The GTI, for contrast, makes 200 hp, and can't burn a whole lot more gas no matter what you do it. Drive an ST using no more 80% of your power, and you'll get good mileage, too. (But that's not why you bought it instead of the PZEV Focus, was it? :))
The cleverness in the EcoBoost is that it saves you gas for all those times you're not going wild on the throttle, unlike say a big V8 in an 80's Mustang. Power when you ask for it, efficiency when you don't, and you didn't have to commit to only one or the other of those forever when you bought the car.