How much do you want to spend?
I wouldnt bother with an induction kit unless you need over 400bhp and a front mount intercooler. The standard Subaru air box is one the best factory standard air boxes ever designed and the STi filter or K&N give the best perfoemance figures. So IMHO get a STi filter in it and leave it.
The dump valve standard on a Scoob is again one of the best designed Dump valve on the market. The VTA (Vent to air) dump valves wont increase you BHP but increase you street cred. I run a Baileys VTA and it sound nice (not too loud).
My recomendations for performace is a decat full or at least a downpipe, then get it remapped at Prosport. This will give you anywhere between 40-80 bhp with supporting mods.
Also get the prodrive handling upgrade, it makes a big difference.
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