I don't think you can get a "short ram" intake on a wrx with the stock setup.
You also cannot run "no intake at all" with the stock mas.
There are CAI for turbocharged cars...but whether that makes a difference or not is one of the on-going debates.
It's worth getting on some cars. According to some of the US based wrx tuning companies, they advise not getting an intake until you have the ecu modified to work with it. If you ask me I think that's a bunch of horse shit but I really don't have much to back that up, they supposedly dyno tested it.