When I do over 85% freeway driving at crusing speeds (re: 75mph-ish and not accelerating on onramps as fast as possible) I can see between 25-28mpg. When a tank of gas consists of mostly city type driving (re: spirited) I usually see between 21-25mpg.
I should say that my driving style is to time traffic lights (re: maintaining/changing speed, be it faster or slower, so I don't have to come to a complete stop any more than necessary).
These are averages that I have regularly calculated by dividing tripodometer mileage by gallons of gas pumped. My 2002 engine internals are stock and my turbo is a EVOIII 16g.
In my opinion, the WRX is really not that bad for gas mileage but it all depends on how heavy your foot chooses to be (and how many times you must come to a complete stop).
70 to 100 faster than you can say "whoooossshh!"