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I think that you will get about 1/4 psi per turn of the rod. However, on the 90- cars, there is a good chance that what you really need to do is adjust your throttle plate. Because the system measures the amount of air flow with the Air Mass Meter (AMM), anything that restricts the maximum air flow will limit the power at full throttle. I have found that many of the throttle plates do not open fully when the accelerator is floored.
Have someone sit in the car while you watch the throttle spool (engine off). The spool should be at the idle stop with foot off the accel. Floor the accel and check that the spool moves to the full open stop. If not, investigate to see that cable is adjusted properly and that there is nothing interfering with the accelerator pedal like carpet, mats, etc.
Remove intake hose from throttle body and inspect with strong light and mirror - it should be absolutely clean inside. If not, remove it and clean it. Check that when closed and held up to light, there is a nice even ring of light around the throttle plate.
With throttle body installed, floor accelerator from inside the car, and inspect to see if throttle plate is EXACTLY parallel to the bore - i.e., fully open. Even a slight deviation from this position will rob a lot of power at full throttle.
If it is not fully open, remove the throttle link and carefully bend the throttle shaft lever toward the front of the car. Just a little! This will effectively shorten the length of the lever and the same amount of movement of the throttle link rod will move the plate further. Check it again and repeat until it opens fully. Note that if it opens more than full, it will rob power just as if it didn't open fully. Dirty injectors is another source of lost power. All injectors must have good patterns and equal fuel flow to keep the power balanced between cylinders for best power, performance, and fuel economy.
Lastly, if you wish to increase boost, I highly recommend that you either use the Volvo "Turbo +" kit which will adjust the boost curve so that you do not get too much boost at low rpm, and give you higher boost overall, or install the Saab APC system (see the directions on the Turbobricks site). Simply increasing the boost with the control rod will increase the tendency to predetonate or ping, and the control system will back off the spark advance negating any gains you might have made. There are also lots of other things that might be going on that can be mis-interpreted by the knock sensor as predetonation with the result that the timing is backed off and you get less than maximum power. It would be wide to check the engine thoroughly to make sure that everything is working properly. Also, be sure to use high octane gas, and if your area adds methanol or MBTE, I recommend use of a fuel additive like AMSOIL PI to keep the fuel system clean and improve the performance of the fuel.
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