If you're looking for muscle, the N/A isn't where you'll find it. There ARE performance parts but its not worth it for the little gains you'd get out of it. Any safe mods are modest at best without beefing up internals, going with forced induction and plenty o tuning.
Tuning is a big deal but yet again N/A imprezas' have limits.
Just be glad you got a reliable AWD daily driver that won't get stuck in the snow like the jerk in that accord in front of you when you hit the slopes. I was that jerk once