I would just go rougly 2-3 psi higher at most, over stock.

Anything more, the turbo is just going to spike and drop in pressure (the turbo is just too small and literally runs out of breath). Any higher than 2-3 psi, you're just putting unnecessary strain on the compressor and prematuring it's life.

If you want to boost harder, get a larger compressor, fuel, and the necessary management tools and gauges.