The only difference you would feel in the shifting is if the bushings used were different. Both in the "H" bracket attaching to the chassis and the bushing on the other linkage going to the stationary attach point on the tranny. (the round one) The only other way is if the linkage geometery was different.
If the linkage material was different, you wouldn't feel a thing. It's just a matter of using strong material so that the linkages don't bend so much under shifting stresses. (slop in othr words) I have looked at the Sti shifter kit on the Sti site and have seen no apparent differences in geometery from the kit that they have here. I have also compared the linkages in the kit to the linkages already installed on the car and they are identical.