We installed a Whiteline 22mm adjustable rear sway bar on the STI to try to get the car to rotate a little more readily. The stock suspension, even with the coilovers, has a marked understeer bias, meaning the front loses grip long before the rear. While this is safe, it makes it difficult to hustle the car around corners effectively.
A firmer rear sway bar can make the back end of the car more lively, and Whiteline's 22mm bar (up from the stock 17mm) is adjustable so we can tweak the stiffness at the back. Since we were back there, we also installed Whiteline adjustable rear end links. Installation was straightforward -- you could easily do this to a car on jack stands. Note that the sway bar kit comes with a set of sway bar support brackets (the gold-colored bar in the picture), which tie together the sway bar mount and the rear subframe. We think this is a great idea -- the stronger sway bar will put a significant amount of stress on its bracket. You can buy the sway bar supports separately if you want.
One note on the installation: The bar has a Whiteline label on the center portion. You would THINK that the label would face upright when mounted on the car. At least in this case, you'd be wrong. Putting the bar on so it'll clear the exhaust meant the label is upside down. Who knew?