The STI comes with rubber shifter bushings intended to isolate the shifter from engine vibration. While they do accomplish this, they also make the shifting action vague and rubbery.
Kartboy bushings reduce this numbness by replacing the factory rubber with 95A durometer (that's firm, for those of you who aren't materials scientists) polyurethane.
Installation takes about a half hour of work; the bushings install under the car, and the car must be on a lift or jackstands to do the modification.
This six-speed kit fits all 2004+, including 2015+ WRX STI.