While your obviously looking for a "14psi is the max you should run" type of answer, I can't give that to you.
What I can do, however, is explain what parameters you need to keep track of as you raise your boost. You will need some kind of datalogging equipment- there are a few models available that run from a palm pilot, and then there are many that run on a laptop. Without having datalogging capability, you cannot know for sure that your car can actually handle any boost increase.

Once you have your datalogger hooked up, you want to keep an eye on a few things as you crank the boost up;
first and foremost is your knock count- if your motor starts detonating, the ecu will pull timing, cutting HP production.
Next, keep track of your oxygen sensor voltage- if this starts to drop, your fuel system cannot keep up with the turbo, and your "leaning out". (too little fuel, too much air) A lean fuel mixture burns hotter, and if it gets hot enough, it has the potential for melting piston tops.

Once you have the ability to monitor those two sensors, you should be able to set a boost level which wouldn't be detrimental to the long-term operation of the car.
If you have absolutely no desire to monitor your own car, I suggest taking it to a comptetent tuner, and have it professionally tuned on a dyno.

Having a datalogger also helps you in diagnosing future problems with the car- rather than pulling 'trouble codes', you can look at real-time sensor data.

Do a google search for "PDA datalogger"+"Evo VIII" and i'm sure you'll find a couple compatible with your car.