Not many people running juice through their Subies. I did some basic research on it awhile back, so I might be able to give you some general info. Id say a good place to start would be a 30-35 wet shot. On a stock turbo/block WRX it should give a nice power gain. From there you can work your way up to a 50 shot. Beyond that and reliability might be questionable. Im not sure if youll need to upgrade the fuel system at all.

Search the various Suby forums or even ask a tuner like SS what he thinks. Im sure he could recommend a setup and even tune it for you.