A common mistake people make when upgrading the intercooler on their Subaru is not fitting a hood scoop splitter to suit. A larger intercooler has many benefits, including that the large surface area can spread heat for effectively. But if the stock splitter is in place, the intercooler's efficiency is decreased majorly.
Another mistake is to remove the splitter all together. This is a bad idea because it then allows for the air flowing though the hood to then go around and miss the intercooler all together. This then results in the intercooler soaking up heat also causing the intake air temps to rise, therefore reducing power.
The best solution is the Process West intercooler splitter, which purpose is to form a complete seal around the intercooler, forcing cooling air through the core, lowering intake air temperature and increasing power. This splitter is designed specifically for the Process West Top Mount Intercooler.