JAS-25T, If you want to increase the boost my first suggestion is to regap your spark plugs to 0.8mm. Doing this with the standard ECU should allow you to run about 10-11psi. After that, as others have already mentioned, "overboost protection" sets in and misfire problems occur.
I upped the boost with a bleeder (to about 9psi) and ran into misfire problems above 4500rpm. I then regapped my spark plugs and could run 10-11psi without it being an issue. However, the real gains from the boost increase were noticed once I got a hold of a chipped ECU which has far better ignition timing than standard. Boost comes on earlier, more midrange responsiveness, and redline is acheived quicker. I can now also up the boost until the turbo goes bang I'm currently running 13psi.
Regapping the spark plugs to 0.8mm can help (and is cheap i.e. $0) but better engine management is the real key.... if only the stock ECU could be reprogrammed....