I just installed the factory speaker upgrade from subaru in my 05 STi. It sounds great! For those who want to keep it stock this is a great option! Magnets are 400 percent bigger (stock ones were the size of a silver dollar) and the popping that is common to subie speakers is gone!
We buy lots of parts from subaru and my parts guy says that the stock HU goes out frequently, and they DO check for aftermarket speakers. So this is a warranty safe alternative.