Stock boost pressure of the D24TIC engines was around 13-14psi (IIRC) on the same head gasket as the non IC TD motors, and there should have been some engineering safety margin in that state of tune, so you would be good at least up to there and a little beyond, IF everything is in good condition.
However there are a lot of variables that go into your questions so the answer might not be that simple. It is hard to say exactly what pressure is "safe" -- depending on some of these factors you might be able to run quite a large boost pressure increase without having issues with the HG, or, the headgasket might not survive almost any boost pressure increase at all.
For head gasket integrity the real question you are asking is not how much intake manifold boost pressure you can run. The critical qustion is what peak cylinder combustion pressure and temperature you can run. Boost pressure does play a large role in determining peak combustion pressure, but it is also affected by:
- injected fuel quantity
- injection timing
- valve timing
- fuel cetane rating
- injector condition and opening pressure
- intake air temperature
- exhaust backpressure
- etc, etc, etc.
And, how much pressure the HG can withstand is also affected by the age and condition of the stock head gasket and the rest of the engine. If it has a stock head gasket and bolts but the gasket is in like-new condition and installed in an engine with a perfectly flat cylinder head that has never gotten hot, then the stock setup can handle a lot of pressure and power. BUT if the engine has already been overheated a few times and has a 30 year old fiber head gasket in it, then it might already be on borrowed time.
Sorry that is a long answer to a short question, but that's how it goes when you are trying to find the limits of something like this. Every situation is different and the only way to really know what you can get away with is by installing gauges to monitor all the relevant values and seeing what the engine does.
I think the intake manifold boost pressure relief valve is set for 1-2 psi above wastegate pressure. From what you saw on the gauge it might be popping open. But it also could be a sticky wastegate valve or a leaking boost signal hose on the turbo. I would suggest checking those things and try increasing the spring preload on the pressure relief valve (flathead screwdriver in the end of it, turn clockwise to increase pressure) and see if it changes anything.
Good luck and let us know how it goes!