If the intake system is as gunked up as you claim, it's possible that there's gunk in the line between the intake manifold and the ALDA which is the device that tells the injection pump to add fuel because the turbo is pressurzing the system. If there's no additional fuel, the turbo's boost is inconsequential.

There's a line that goes from the intake manifold to a small black device (overboost swtichover valve) on the firewall, then from the valve to the ALDA unit atop the injection pump.

Clean both lines and see if there's an improvement. If not, don't worry, there's more to check.

TEMPORARILY bypass the overboost switchover valve. Don't run it with the pedal to the floor but run it hard enough to see if there's an improvement. If there's an improvement, then clean the valve. It should be safe to spray brake parts cleaner through the valve, but take that advise with a grain of salt. The valve is open when unpowered.

Hopefully that's it. If not, attach a pressure/boost gauge to a tee fitting in the line between the intake manifold and the valve and repeat with the gauge tee'd in between the valve and the ALDA. See how much boost you see with the car on the road. The turbo doesn't make boost without a load. You should see boost kick in around 2500rpm and a max of something like 12psi. This should tell you something about the health of the turbo. If you see boost before the valve and none after the valve, it could be that the pressure sensor beside the intake manifold fitting is stuck.

I hope this helps.

Sixto
91 300SE
87 300SDL
83 300SD