Welcome.
All-wheel-drive Subarus will never be amazing, high-mpg machines, but they're small and aerodynamic enough that the fuel economy isn't terrible, either.
The most dedicated Subaru aeromodder here was Istas, and you should read through
his build thread on his car, "Car". Too bad we haven't seen Istas since August.
If you don't like the look of unpainted sheet metal riveted to your car, you can use coroplast (corrugated polypropylene), attached with zip-ties.
As for my Subaru, check out my fuel log. Small mods and careful driving brought me from 22 winter / 26 summer to where I am now.