Personally, I am not into performance modding so you'll probably get better replies from others who are.
Assuming you have done a proper stage 0 service, the limiting factor is likely to be fuel or air.
Your B230FK probably has an over-boost function built into its ECU. If it detects too much air for a given engine RPM then it will cut the fuel. This is more like a sudden loss of power rather than gradually hitting a limit.
If the loss of boost is more gradual then something is probably restricting air flow. What sort of condition is the turbo inlet hose in? If it's too soft it might be collapsing under certain conditions and restricting air flow.
I think it's also possible to hit a condition where you're passing so much air over the AMM that it cannot draw enough power. This is unlikely at relatively low levels of boost, but you could try carefully cleaning the AMM if you haven't already to restore its performance.
Good luck!
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