Do you have the stock tubro? If so, then everything should be fine. I bet you can run safe with 18psi (but at that point you'll be blowing nothing but hot air) so, I recommend 16psi. I dont think your small stock turbo can push your stock fuel system to the limit. You may have difficulties reacing 16psi withouth a boost cut defender - Im not familiar with with the "stock" limits of pressure with the WRX. So, you may also want to invest in that.
I hope this makes sense to you guys. I just got my wisdom teeth pulled about 4 hours ago.. and im still feeling kinda free....
Wwwwweeeeeeee