Originally Posted by xluben
I'd worry more about your motor and your trans
Motor for sure.
I have seen great results off of the stock STI intercooler. I think people generally want a new intercooler for looks at first. There are gains to be had but there are also other ways to make nice power. The WRX intercooler is known to separate on the edges and heat soak pretty bad...but the STI one does pretty well. Turbos like the 20G or the 1.5 are designed to be stock location turbos. You won't lose anything but you can gain some by going to a new intercooler.
That said, I did make nice gains going to a FMIC on my setup. How much you gain will depend on what else you are doing.
BUT...it's a Subaru. The motor is not if...but when.