Upgrading to a bigger intercooler will have minimal effects with the stock turbo unless you have engine management changed to take advantage of the new intercooler. In other words, if you get a new intercooler you'll make more power with an ECU reflash or replacement than if you kept the stock ECU.
If you want more power for a decent price...I suggest an exhaust system, a better air filter, and an ECU reflash to make sure all these components are working at top efficiency. If you don't care about your warranty or have a lot of money to drop on installation/reinstallation, get the uppipe/downpipe combo as those two make a substantial amount of power for the price. Downside: warranty is void if Subaru ever finds out, cops, etc.