Hi there. I've got a milltek cat back on mine. My subaru dealer is also a milltek dealer and he says a cat back will not affect your warranty. I can highly recommend the milltek the cars are just too quiet as stock. I was over at the Isle of Man for the TT last week and they use the new sti's as course cars, these things were coming past us flat out, 5 or 6 of them at a time and you can't hear any exhaust noise at all. For me the milltek just compliments the car and let's you hear the motor working without being too loud. I bought mine through Stan palmer subaru at carlisle.