I was also start thinking that there has to be some kind of limiting point in exhaust side.
So next step is measure back pressure after turbo. If that presssure curve rises on same time when MAF value dropped we know reason. BTW. I just remember that last owner has drove almost 6months with huge boost leak(that EGR hose). So that can be reason why now gas flowing seems to be limited...
I am already thinking next stage for getting better performance. Any already tested info when i need to be adjust rail pressure up and start tuning also other MAPS(SOI and others) ?