I have owned both cars.
Last year I owned a 3.0R Spec B, and now currently own a 2.0GT Legacy wagon.
Both manual.
I can honestly say that the 2.0gt is more fuel efficient off boost, period.
But putting your foot down, the turbo becomes extremely fuel inefficient.
This is because turbo engines must run a rich, high fuel mixture when boosting to prevent knock and excessive temperatures.