[Turbowned]

Yeah, this is way too vague of a question. I'm going to assume you have a garage full of tools and know exactly how to work on every facet of your car.

-Buy/install a turbo kit and exhaust system. There are plenty; pick your poison.
-Upgrade to coilovers, adjustable control arms, and bigger sway bars. Corner balance and align.
-Upgrade to AP Racing front brakes, Motul RBF fluid, upgraded rear pads
-Remove rear seat, spare tire, and anything else not bolted down when you go to the track for temporary weight reduction
-Lightweight battery, carbon driveshaft and lightened flywheel for permanent weight reduction
-Lightweight 17-18" wheels and sticky tires. Again, pick your poison

Now go to the track, shake it down, and determine what changes need to be made. Adjust suspension, tuning, determine if aero upgrades are needed. Play around with the whole mess until it works. You're likely to spend between $15-20k for a host of modifications that will make your BRZ a well-rounded 350whp machine, if you buy decent stuff and install and adjust it properly yourself. Add more if you need someone else to do it for you.