Gasp, there's no such thing as free power?
Improvements in combustion efficiency (e.g. DI), aside....it simply takes a given amount of power to accelerate a car at a given rate, maintain a given car at speed, etc. Without a large discrepancy between the overall efficiency of a the turbocharged engine vs. a larger N/A powerplant (which is there isn't), resulting mileage will be pretty darn close all other things being equal.
"It's not what you drive, it's how you drive it"
****, i think I just quoted Clarkson....