So it may be worth starting with the basics, has the inlet been checked for restrictions & is the air mass ( if fitted) reading correctly?

Has the exhaust been checked for restrictions, is the EGR valve closing as it should.

Has the turbo been checked for play / clearance?

Has the supply for the control valve been load tested, is the PWM signal being pulled to earth ?

What pressure do you have on the supply side of the control valve, how much of it is getting though the valve with a high duty cycle applied?