If you drive your 2004-2007 STI competitively in traffic, or at the track then you cannot achieve maximum handling bliss without a set of Kartboy Rear Drop Endlinks. Your rear endlinks are the main point between the sway bar, and rear lateral link. Your factory rear end links are made out of a soft material that will weaken over time.
The Kartboy Rear Drop Endlinks are CNC cut, and hard anodized 6061-T6 aluminum with stiff urethane bushings for the right amount of dynamic flex. Stop knocking down all those cones at your local autocross events!