i have a 300 sd turbo but I have not touched the turbo... I have
just finished replacing a turbo on a saab. I have done 3 turbos so i
have limited experience. You waist gate is designed to open and let
the exhaust bypass the turbo to keep it from producing to much boost.
So on a saab the computer opens an electronic valve on top of the radiator and it allows vac to flow to the diaphram that pulls the waist gate open.
The benz would have a simular set up. If your waiste gate were stuck open then you would not have any boost and you would have decreased performance. Most turbo inspections occur by disconnecting the exhaust. You can visually check the waist gate then (see if it is stuck open or closed) and you can stick your finger in and wiggle the turbo fan shaft. (you are looking for excessive play) which indicates the bearings are going bad. You can also check the exhaust area for oil which indicates a bad seal.

But First,
Change both fuel filters,
clean the banjo bolt on the back of the engine.
clean the strainer in the fuel tank.
adjust the valves,
Do a diesel purge,

save the turbo for last....