I previously had a very low mileage 56 plate wide track STi Type UK with PPP and the only mods were Prodrive spring kit, Prodrive 18" PFF7s, Pipercross foam panel filter, Hayward and Scott straight through rear centre pipe and a forge VTA dump valve. Fantastic, frantic performance and huge noise on full power. Probably the best of all our 5 Imprezas, but couldn't afford to run two. Remaining Hawkeye WRX has been with us for five and a half years and is also a great car. I have always kept the standard airbox and found Forge dump valves to be fine. Enjoy your first Scooby, but treat with great respect until you are used to it!!!
John