Depending on which intake you get, what boost level, the temperature outside, elevation, timing, etc., etc. Its very hard to even guess what gains could be seen honestly. Again the shop should be able to answer that as they see it more specific from them to begin with.
As far as more power and better gas mileage going together, yes it can happen. Basic principle of more power less work for the engine which can get you better MPGs if you have some self discipline. At my power levels in town I have been able to average about 22MPG, and I still get in it once and awhile. Now I can also go as low as about 15 MPG on the same route if I dont want to obey certain rules of the road.
Keith, Dom (Maxwell Power/DTRacing) would be great for an E-Tune. I wouldnt recommend the OP drive there as its no where near him. But again, Dom is great guy also.
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX