Hey mate.

Depending on the tune it's not uncommon to have better fuel economy than you currently have. Most of the time it depends how heavy your right foot is. Every car is different.

Sounds like you've been doing a bit of searching around, so you'd probably know that if you were to retune your ECU (Maximum Motorsport = ECUTek), you'd want to finish off your exhaust to a full Turbo back system first.

There is a thread on here which shows all the recommended workshops in Perth who can do this for you. While Maximum's are alright, they can be pricey. Recommend a Verso retune for $990 at All Star.

http://www.perth-wrx.com/vb/mechanicals/6775-perth-workshops.html