I am curious about this too. I own a '05 with the extended warranty. in my case I wanted a conical injet air intake system but some told me that it my engine star to act strange I could not go to Subaru and get to fixed for free because they will claim that thatís not the air intake that the car had from factory and it is not a Subaru part.
I opted for a "deleted intake-silencer" mod and I will buy a true-flow air drop-in air filter. I would like to do more but I am afraid to void the warranty