Your WRX comes with side cowling stays (located under the front fenders), but they're made of inexpensive stamped steel that's not very stiff and not very light. These lightweight aluminum versions have four times the stiffness!
Installation is simple -- you can reach these parts by just unfastening and pulling back the front wheel well liner -- but small hands and a ractching box wrench set are helpful.
Set of two side cowling stays. Fits all 2002-07 Impreza/WRX, including STi.