First make sure the car is up to date on all required service and everything is at least factory spec. Fluids, belts, filters, everything. Prefer authentic Subaru parts.
You could always stop here and enjoy. No? Reconsider. Still no?
Then start on the ground. If your climate allows it get a very sticky set of summer compounds on whatever wheels the car has right now (don't buy wheels, it's a waste - the car already has wheels). This will make the most difference. The harder you can turn, brake, and steer, the quicker the car will be.
Next just work your way in and up. From the wheels look at brake pads, next look at the strut inserts, next the sway bars, next the springs.
The car already makes adequate power. You can stop there IMO.
If not, look at a management system and an up pipe. Keep in mind replacing the up pipe will violate federal emissions restrictions.
No matter what, add power last -- if at all.