Well the ecoboost is a direct injection motor. If you keep the same engine architecture (injectors, turbo, and such), you won't see any MPG droppage unless you floor it all over the place. At least that was the experience when I owned a saturn sky redline. Many owners that had the power upgraded reported increased fuel mileage as well because of optimized mapping of the other parts of the fuel curve by the tuners. I think with the stock injector setup, they were able to go from 260hp to around 330ish hp. I'd expect the ecoboost to hit probably around 400hp before needing either bigger turbos or larger injectors.