this isnt exactly true either. you want to keep it in vacuum but with pedal to the metal. This keeps the fuel being totally used for energy. so hit the gas hard and shift early. You dont want to creep up the rpm's pulling 0 boost b/c then the fuel doesn't totally get burnt. Do some research on hyper-miling and you'll read what im talking about.
saying all that junk, i got around 17-25 in a regular 2.0l wrx. its that all wheel drive that kills you. new plugs, diff oil, clean air filter and free-er exhaust had no effect on fuel milage for me...it just chewed up gas!