Well, you are right about the WRX running 12 psi, but you are wrong in the fact that the wastegate doesn't open @ 12 psi. The WRX uses an advanced method of boost control to vary at different temps and altitudes, therefore the standard internal wastegate isn't the only device used to control boost.
The WRX internal wastegate open, I have seen, anywhere from 4 psi to 8 psi. I have a friend with a 1.8L L Turbo, and his opens around 6 psi. There is another guy, Darshu on i-club I believe, and the wastegate on his WRX turbo doesn't open until 4 psi. So, as you can see, it differs. Kind of strange isn't it?
My friend who is a Subie tuner/technician vaccum tested a WRX turbo and the wastegate was measured opening @ 8 psi.
So, my advice is put the turbo on and see what happens. You DEFINETLY won't get 12 psi, but it could be as high as 7 or 8 psi.
I know others have had similar complaints about the WRX turbo, but no one can seem to find new wastegate springs for the wastegate's.
Graham