I know Rick was asking me about this earlier so I thought Id just post since other people may have the same questions...
Not to sure about WRX, but...
98-01 2.5 RS sedan factory states it has 4, 6.5" speakers, 2 in front 2 in rear, front also have tweeter mounts if the premium audio was installed in the car...
I have 2 Boston Acoustics pro series 6.5" in the rear, they had to be semi custom installed due to the shape of the firewall... they fit fine
I bought a second pair for the front and here is where I came into problems... the manual states 6.5" however, the stock speakers are garbage and are only about 1.5 -2" deep... most aftermarket speakers in 6.5" are about 3-3.5" in depth... the speakers would not go.... so I had to move down a full inch to 5.5 in order to fit the speakers...
If you want bass, and dont want subs, Boston Acoustics are perfect!! any other questions ill try and help to the best of my knowledge...