Chch wise buzz Tony at RDL Performance as im sure he can sort u out, Me, Koom and many others from here only go throug him or Stubaru's. Ul honestly be only needing to purchase a bore muffler in essence to get the noise.....performance gain won't be great at all in a non-turbo.
But do get use to having ringing noise in ur ears after any decent road trip..... I use to have a big bore on my previous wrx, I miss it with my current wrx in some ways but I don't miss having my ears ringing after a 3-5 hour road trip haha