there appears to be a "rule of thumb" that 12 psi is the peak for standard intercooler, and 14 psi is the peak for fmic on standard gtst-t ceramic turbs, in order to run with any degree of safety before something goes bang.
that said, I am running 9 psi, and many others are running a lot less than the above quoted figures. Draw from that what you will.
the difference in rwkw between 12 and 14 from gt25t seems to be very little, although there is a significant increase in torque.
expect 250-300rwhp. I cbf working out what that is in rwkw.
also, find the search button cause these sorts of questions have been asked a bazillion times before.