get some seat time first and get used to the car.
Personally, I'd concentrate on handling and braking first. N/A cars are tougher (and often more expensive) to squeeze for more power.
making it handle and stop better makes the car more fun to drive. Perfect example is my previous car, 07 corolla.. gutless.. however once I did the springs/wheels n tires it was like a gocart. Still gutless... but not having to slow down for corners makes for alot of fun and the less you have to get on the throttle to speed back up on exit.
sways, links n springs aren't expensive bits for the impreza's.. thats where I'd start... and sticky tires.
lighter wheels will also improve how the car accelerates without even touching the rest of the driveline. More expensive.. but something to think about.