I am no expert but I have read plenty on iwsti.com and nasioc.com on the subject of CAI for MY06 WRX/STI. My gut feeling is that it's a waste of $$. Best money spent would be flash of
ecu like ecutec. If you want you can also look at getting rid of the snorkus (intake resonator) on the factory intake. Carefull with oiled filters as they can cause you problems with oil on
MAF if you are not careful, notice I did not say "will cause you problems". Best mods in my mind is the exhaust to free the boxer beat, a bit of suspension work such as whiltelines sways, endlinks, lowering springs or if you have the $$ then coilovers. Once you have these things in place then the flash on
ecu and you have a new and vastly improved ride over stock.