when i first bought my car it was pretty standard, am now running FMIC, full 3" exhaust with nismo dump and front pipe, SAFC and turbo smart boost controller, and am now running a very nicely tuned 251rwhp. 11psi.
have heard that the standard turbos are good for around 13-14psi, anything above that and you risk damage to the turbo, ie: disintergrating the turbo exhaust turbine in my case. My standard clutch also had to go, wasn't worn just physically couldnt clamp hard enough.
also just a note, my housemate also owns a r33 with a similar setup, only he managed to score a Mine's ECU (i'm o so jealous) and at 13.5 psi, it's good for 301rwhp.