I installed the WRI STI 19mm front swaybar (purchased from my local Subaru Dealership) on my 04 STI. The change is subtle, but here are my initial impressions.

The car feels more satisfying and predictable - it no longer has such a non-intuitive transition from neutral thottle (push) to on-throttle (near oversteer). Tire wear seems to be improved as well; the back wheels take
more stress.

I tried the setup at the Streets of Willow road course, running in the reverse direction. 37psi F and 35psi R worked well with the stock tires [0 toe F/R, front camber maxed at -1 deg, rear camber stock]. Tire wear was very well distributed, despite the lack of front camber. The rears were evenly heated and the fronts were slightly hotter on the outside edges.

I have done about 8 track days ('98 M3 and a '97 Legacy Wagon GT). This is my first time at the track in the STI, and the first time I had driven this particular course. From the posted times, I had an unofficial lap time of 1:36.4, about 1.2 seconds better than other cars w/ street tires (including 4 Evos).