Compare the STi to just a KB SS? Well, like a few have said, the STi one replaces the whole shifter unit with one of an extremely short stroke and a smooth, yet has a "notched enough to let you know it's there" feeling. You have to remember that the KB SS is just a replacement shift lever, not a complete assembly. Stock (even with the KB SS) is like rowing a canoe, while the STi version is like being on a rowing team, short and precise. The STi shifter makes gear changes effortless and almost feels like a sequential manual, minus the fact there is still a shift pattern.
Reason I know? Short throw shifter in 6MT in RSTi FTW!
-ryan