The only thing good info you provided were sway bars and tires.
After driving my car for a year, I can say that the sti doesn’t really need a short shifter. Bushings would help though. This is all personal preference as I’ve seen people take short shifters out of their cars before.
Corey I know you are biased towards the AP because you put one on your car and it got slower…but, I think the best power mod for him right now would be an AP stage one map and a catback. Easily 40+ chp and 30+ ctq. This could be had for under $600 dollars if you know where to look. Also, you could install one without the other so your but dyno wouldn’t have to wait.