We installed a K&N filter on the Project_STI vehicle and noticed that the engine breathed easier, but the noise from the turbo spooling up and the operation of the bypass valve increased. Those are pleasant sounds that most drivers like to hear from their engine. Bottomline is that there is a noticeable improvement (seat of the pants) in how quick the turbo spools up and the car accelerates (significant but not a substantial performance difference).
I swapped out the stock filter for the STi one you linked to. Did it at 7K miles. Didn't notice a "butt dyno" increase, although it does seem that you can hear more "good" noise from the swap. My buddy picked up the stocker and replacement and breathed thru them (a very, very scientific method for those of you unfamiliar - :lol: ) and said the STi logoed one seemed more free flowing. It sure does look nice - too bad the STi logo is hidden under that cover....but I guess it says "STi" in enough places on our cars already.