Well, it's pretty simple to check your fuel pump. Log 230 on vag-com and see requested vs actual. A drop in power at 4000 RPM could be from your fuel pump. Fuel preesure sensor is limited to 130 bar (I have AXX too) but even if it's limiting the pressure a little bit, you won't feel it. If you get to feel the loss of power, then it must be a massive drop (like 10-20bar) !