Just installed the STI Short Shifter plus Kartboy bushings a few days ago. In short this is a great improvement and makes driving a lot more fun.
I purchased the shifter from subaruparts.com for $200 and the bushings from Kartboy for $35.
The STI shifter replaces the entire shift assembly. The shift rod as well as the shift lever are very different from the stock pieces. The rubber bushing looks to be a little firmer than the stock bushing but that is irrelevant as I replaced it with the Kartboy piece. The rest of the assembly looks the same as the stock pieces.
Installation is very straight forward and anyone comfortable with a socket wrench should have no problem. Unlike most of the aftermarket shifters you have to get under the car to install this one (if you are replacing the bushings you have to get under the car anyway). I wasn't sure about getting the stock bushing out to swap in for the Kartboy but a good firm poke was all it took to get it to pop out. I don't have a total install time as I already had the console apart to install the alarm upgrade. Expect to spend about 45 minutes to do the job. Two of the 12 mm bolts are a little hard to get to and having a 12" socket extension will make life a lot easier for you.
Once installed you will immediately notice the difference. The 40% shorter throw number seems accurate. Shifting is also much smoother and much more precise. You will notice a very small increase in NVH. You can now feel drivetrain vibration in the shift knob. The vibration is not bothersome, just noticeable. The stock shifter has a big rubber dampener in the shift lever that isn't present in the STI unit, also the solid Kartboy bushings replace the STI rubber mounting bushing. I imagine both parts contribute to the increased NVH.
I highly recommend this mod. Subaru should put the rubber dampener of the stock unit on the STI Short Shifter and install it as standard equipment on the WRX.