The wagon front lower control arms and rear lateral links are what space the knuckles out from the car. The WRX ones are more narrow and made of a different material than the STI counterparts. You can run stock WRX lower control arms and rear lateral links along with your stock sway bars and be just fine, you don't NEED to replace any of the suspension stuff aside from struts and springs. STI struts and springs will bolt right in to your wagon but you will have saggy ass. If you want to upgrade to the STI stuff it will give you a wider track, which is awesome, but it is not needed. If you do decide to upgrade lower control arms up front and lateral links in the back you will need to change a couple other minor things around. In the back you will need rear STI end links. You can keep the same bar I think but you will need the "I" style links instead of the "C" style links on your WRX now. Up front you have to get a whole new bar because the STI bar is wider. You will also need end links.