Don't know about the shorter impreza shifter, but I got the kartboy kit with the forester shifter. Overall I like the feel, but it can increase noise and vibration. If you use the stock knob or a heavy knob it probably wouldn't bother you. I like a really lightweight knob (45g I think!) and NV bothers me some with the kartboy shifter. If I had to do it all over again I would research the STi short shifter linkage compared to some of the other ones. The picture I saw of the STi looks like it has a rubber-damped shifter like the stock one. Less NV but maybe also less direct feel.
I wound up taking the rear kartboy bushing off. it was a real PITA to try and install and I partially messed up the threads on the car but had a tap to chase them. I think the material shrinkage makes the bolt holes not line up properly. Also, the rear bushing does make it sit higher in the car but it also pushes up on the rubber boot protecting the shift linkage and I didn't like the way it looked, like it might cause it to crack or fail prematurely. I put the stock rear bushing back in and like this setup better. The knob is a little lower and it still has a more direct, "notchy" feel I like in combination with the front kb bushings. Couldn't tell much difference in the feel with the rear stock bushing vs the kb bushing.
One of the manufacturers (cobb?!) makes an adjustable one which I thought was a neat idea but found out about after I got the kb.
Good luck deciding!