Hey i saw your post, your on the right track with getting all that paperwork for the turbo and supporting mods. What year is the car? Because on the younger WRX's if theres a bigger turbo you absolutely need a bigger fuel pump, bigger injectors as well as a tune.
Id really reccomend you get a basic maintenace shop or someone specializing in imports to do a basic used car inspections. Its around $50-70. Message me back with some more details.