Hey im new too!! lol.
Alright man first off let me just say that i am in no way a WRX guru i recently purchased my 2011 WRX a week ago and it is also my first forced induction car. With that said i have been on the on/off track racing scene for a while and have owned and tweeked several N/A performance cars prior to my WRX. So now that you know the background i am working with I will attempt to ASSIST you with your possible problem. The number one thing i can say is that you should definitely feel at least feel the car pull harder when the turbo spools up. The WRX you own does not have the same turbo as my 2011 so a comparison would not be accurate since my car has a 2.5L engine and a VF52 turbo which as far as I know makes abt 13.5 lbs of boost stock. When I push the gas to WOT there is about a 1 second period of turbo lag and than there is no doubt that the turbo has spooled and is at max boost. If im not mistaken your subaru is equipped with a 2.0 liter engine and i believe it is called a TD04 turbo that should make about 13.5 lbs of boost as well. As for the gas problem if you really used a quarter tank in 50 miles that is not normal at all. But remember dont just assume you guage is accurate maybe look up how many gallons your tank holds, fill it up and divide the miles you drove by how many gallons you really used to see if its a faulty guage. or avoid it all and take it to a subaru certified mechanic and have them look it over for you. They will charge a small fee but it is worth it so you know what you are getting yourself into man. hopefully this is helpfull.
P.S. if anyone sees any incorrect info in my post feel free to correct me as I am a new guy on here, thanks yew