It definitely IS the spark plugs. It seems that on high boost the sparks are designed to go off prety late and that means the trigger minimum voltage must be much higher and the lean mixtures are prone to extinguishing flame either way.
What can I tell you that your problem is complex, and only tells us that the engine manufacturer tuner has done a VERY shitty job designing the control tables. This wouldn't happen on an old Honda or an old Rover, those are designed to race.
If i disconnect it, i can run as much boost as i want without misfires but the AFRs drop in the upper RPM (beyond 4500-5000) from .75 lambda to around .90
It is very much possible that the emergency source of pressure information can't measure pressure over XXXX (or isn't pre-programmed), which would lead it to report a pressure of max. say 1.5bar and adjusting the fuel accordingly. That would cause the lean running.
You want modified silver-wire turbo spark plugs, or the LGS type, or anything with many sharp edges and a catalyst. I had fine results by filing the outer electrodes of silver-core plugs to a special shape to lower the trigger volateg of the spark plug. But from your data it is hard to tell by which effect is the flame extinguished (it could be by the petrol itself, by injection timing, etc?)
Do a run with a lot of misfires, then suddenly stop the engine Take out the spark plug and observe. Is it clean or black? Was it running too cold? Is petrol dripping from it? etc, etc, etc. Which cylinders are doing this? All? One? Some?
"It is not the spark plugs or ignition" is an excuse that says it IS the spark plugs and ignition, but you don't know how.