The whole reason for the turbos on the Ecoboost engine are to fool a small displacement engine into thinking it is a large displacement engine. Same for turbo diesels.
The Ecoboost's low speed efficiency comes from its small displacement not its turbos.
You can never defy the laws of nature. Fuel energy is converted to heat. Heat is converted to horsepower. How an engine does this specifies how fuel efficient it will be. At low horsepower demand situations a small displacement engine will be more efficient because fuel is not needed merely to rotate large engine parts. In low horsepower demand situations a large displacement engine loses efficiency because energy is needed just to move it's more, and larger parts.
In high horse power demand situations all of the above no longer applies because both engines are working at capacity. In low horsepower demand situations the small displacement engine will focus a smaller percentage of the fuel burned on actually turning the engine parts. In the high horsepower demand situation a greater percentage of the work the engine is doing goes to the actual work being performed.
There is no free lunch. At the end of the day a certain level of fuel will still be needed to move a heavy brick through the air while its tires are trying to resist rolling. This is constant no matter what is under the hood.