With the WRX, since the car is factory equipped to deal with the general life shorteners of turbo, you should be fine running at 18.
I believe the stock turbo runs at less than 15 but the fuel system can probably take a little bit of heat, just be careful.
Get a boost monitor/controller for sure, and you might wanna think about upgrading your fuel system pretty soon...I would hate to see a nice car like that blow up because you overboosted your engine.