I'm sure there are others that have more experience than I do with a modified 930 like yours. The boost is controlled by a spring and diaphragm in the wastegate. It opens a valve and lets exhaust bypass the turbine when the intake rail pressure reaches a point determined by the boost spring. 0.7bar was the stock spring setpoint (at least for my car). A 1bar spring allows much higher boost. This is simple vacuum-controlled mechanics, no electronics involved (yet).
If pre-ignition is what's happening, you should replace the 1bar spring with a 0.7bar or a 0.5bar spring, and add an electronic boost controller. You get an accurate boost gauge for your trouble, and you can set the boost anywhere from the spring setting (even a fancy electronic contoller can't go lower than that) up to 1 or 1.2bar.
In the winter, when it's cold out and raining (poor mans water injection
) you can probably get away with 1bar or slightly higher without problems. In the summer at high ambient temperatures and especially after the motor and intake comes up to temperature ("heat soak" as it is called), you need to keep the boost to a lower level.
Bruce Anderson's book also has a section on mixing gasoline to adjust the octane level. It might be helpful - it is for me!
Good luck!