I put a Perrin on my first WRX after I had modified the intake and put on a Perrin uppipe, and modified the exhaust with a 3" turbo-back with only a muffler. Vishnu did a reflash and then I put on the Perrin pulley. The added power was immediately apparent.
The WRX I now have also has a Perrin pulley, but we put it on when we did everything else; so there's no way to know what effect it might have. I guess what I'm saying is that if you have basically a stock engine, the pulley will make a difference.
Intake: just take the plastic intake tube off, and then take out the plastic resonater or whatever-it-is inside the fender. Cover the hole with tape or a piece of metal or whatever, so that the intake gets air only from inside the fender. Find a piece of rubber or pvc that will attach onto the intake tube on the air-filter box and put it into the hole so that no hot air from the engine compartment can get into the filter box. You will have more than enough air. It will be cool air. It will not be hot air from inside the engine compartment. And it won't cost you anything but a little time.