As above post, I too have the factory short shifter and installed Perrin short shift stop. Installed easily. Works well. And my shifts are delightfully short.
I would like to see the source of the above post that the optional OEM short shifter only reduces throws by 10%, for I have repeatedly heard numbers in the 20-30% range (though I do not have a factual source for that, just repeated, forum, "urban legend").
Subaru states that their short shifter "Significantly reduces shift throw lengths for crisper shifts and sportier driving feel," but again, not yet found a precise/mathematical amount of how much shorter Subaru's is.