Welcome to the board man. Your desires for mods are exactly what most STi owners should do.
To get that 75 horsepower, all you need is a turboback and a reflash, and some will say to port your wastegate as well. If you're serious about how you drive the car (i.e. autocross, club track days) you are definitely in the right place! The only other mods I personally would recommend right off the bat are a larger rear sway bar and some steering rack bushings. Simple changes that make a world of difference in how the car feels and acts, though not necessarily making it faster or harder to drive. After that, you should think through your mods by looking for things to change then finding the right parts to do it, rather than the other way around. As you've seen, it's a rather well sorted out car.