A short shifter is bling. It's a feel mod. It does nothing for performance, because the transmission does what it is going to do, irrespective of shifter throws. You can shorten them to inch-long travel, but the transmission isn't shifting any faster, which means that a short shifter isn't doing anything to shave time, or improve performance. They can feel better.
Also note that hopping in two GTs, one with the short shifter and one without, does indeed verify the 30% throw length reduction claimed by Subaru. 30% isn't a lot. You notice it, but you don't go from one to the other and say "Wow, what a difference."
The stock GT shifter is smooth, but don't mistake that smoothness for a lack of precision. The short shifter adds some mechanical feel, for that "click, click" that feels sporty and precise. But for a shifter to improve precision, it will come with a stiffer bushing, to start with. The STi short throw bushing isn't any stiffer than the stocker. It wasn't on the WRX, and it isn't on the Legacy.
I ran the STi short shifter on my WRX, with Kartboy bushings. Very smooth, but more mechanical than the stock WRX shifter, which is indeed sloppy, unlike the GT stock shifter. Adding the STi transmission mount smoothed out the feel of the STi/Kartboy bushing combo, to the point where it "felt" dramatically better than the stock shifter.
The GT's shifter is a large order of magnitude better than the WRX's stock shifter. Kartboy bushing might add some feel to it. It might also make it feel incongruous with the well-lubricated whole that is the stock GT. Be sure you know what you're after before you go in.
A friend has the STi short shifter, and he likes it. Unless it smooths out a LOT, I don't.
But it is feel, not performance, which slides it neatly into the "bling" category. Sorry.
p.s. No, I didn't move it. I don't have moderator powers, thank the stars.