the AEM kit only has one pulley, the aternator pulley. I tried installing it and the long bolt next to the alternator (on the left) broke when I tried to loosen it. I replaced the bolt and will try again.-
the only pulley that will have a good effect on performance gain will be the crank pulley. The accessory pulleys will only free up a couple of HP at best.
Make Sure that if you get a pulley that it comes with belts or gives you specific belt sizes that will work. I went through hell getting the right sized belts for my "stock size" lightweight Unorthodox Racing crank pulley. Hopefully they have updated their documentation for the WRX pulley. I called them and gave them correct belt sizes and they said they would update their instructions with the info. Thankfully the UR pulley works very well.