If your worried about the warranty then don't go changing parts but go right ahead and get the
ecu re-flashed ECUTek ( what I am running ) or Verso.
A decent tune will make the car much more enjoyable to drive and decent amount quicker.
More than likely Subaru will never know and when the warranty runs out and you decide you want more ( and you will ) change a few bits and the re tune to suit will be fairly cheap. ( Race Torque now only charge a fairly reasonable hourly rate for any updates to my tune and rarely takes more than an hour )