I know that this subject has been beaten to death. However, most of the information is speculation. I installed the intake and heat shield on my 06 WRX wagon a couple of days ago and I thought I would share my first-hand experience.
First off, the fit and finish is excellent. Installation was a breeze. It took me all of 30 minutes. The hardest part was applying the rubber trim to the heat shield, which wasn't hard. The stock box and hose came off easily, and the heat shield and intake went on very easily. The fit was perfect. No problems - as is to be expected on a factory-sanctioned part. The kit comes with the CARB label and three SPT decals. If you are going to apply the CARB label, I would advise that you carefully clean the spot where you are going to apply it. Everything has a light film of oil on it.
I was a little concerned about the sound, since I am an old fart who has to act my age at times. I didn't want it to be too loud, and I wasn't looking for sound effects. As it turned out, the sound is perfect for my tastes. You can hear a little more engine growl, and with the radio off, you can easily hear the hiss of the turbo spooling, and occasionally hear a sharper hiss when coming off throttle from the BPV. It is enough to let every passenger know that the car is a turbocharged performance car, without any gimmicky squeals or whistles.
As for performance, it is probably a bit early to tell for sure, since I reset the ECU during installation. However, I have noticed an obvious increase in power and quicker turbo spooling. It is subtle, but it is definitely there. I hope this gets better asthe ECU adapts. I am still running stock exhaust, but will install the SPT cat-back next weekend.
On a side note, the parts guys at the dealer told me that I needed to reset the ECU after install. They told me to go talk to the service guys about how to do that. I played dumb and went along. The service guys told me that I could not reset the ECU myself, even after I asked about disconnecting the battery, and they told me that I would need to pay them $40 to do it. I asked specificially if they would just reset the ECU or reflash to a newer map, and they said reset only. They said that the battery disconnect would *not* reset the ECU. I got pretty testy about them trying to charge me $40 to do something that I was pretty sure I could do myself, and they told me I could go to AutoZone and they could hook up a reader and reset it for free.
It was at this point that I realized that I know more about my car than they do. I must say that they did not inspire a lot of confidence for any future work they might do.
At any rate, I like the intake, and it definitely will add a modest amount of hp to a stock 06.