The subaru SS is really different linkages not the stick so much, which from my research when i was looking doesn't shorten it up as much as a kartboy SS, for example, or any aftermarket. Also I'd recommend upgrading the front and rear shift bushings, the STI rear short shifter bushing you get with the upgraded subaru SS isn't much better than the stock WRX one, which I know for a fact because I have a WRX, STI and aftermarket rear bushing, aftermarket all the way. Also with the stock SS the front shifter bushings aren't upgraded at all. It is still probably pretty sloppy feeling. With the upgraded bushing and stick you get a notchier feel.
EDIT: Here is a link to some reading about the Subaru SS vs aftermarket
Andy - 12 WRB WRX
Tuned by Innovative Tuning
294 AWHP / 311 TQ