Having read the numerous posts on this topic, and experienced and solved the same type of problem, I must admit that I made the usual mistake of running my engine too low: why speed more than 6 knots while admiring beautifull landscapes, unfortunately without wind (1800 RPM) .
One day, while wanting to go a bit faster, the engine started to refuse to go higher than 2000 RPM.
I opened the exhaust elbow and found it largely restreined by charcoal and the propeller of the turbo litterally completely blocked. It took me hours, with a very smal screw driver to free all that, including the wesgate, but without removing the turbo. The result was far from being perfect, but better: about 2500 RPM.
Recently it went back down to 2000 RPM; it was time for something more efficient : I took the whole turbo into pieces, including disconnecting the shaft of the wastegate from the lever which moves it in case of overpressure (not only the rod linking to the actuator) . Having done that I was able, having all parts in hand, to clean properly the flap and the seat of the wastegate with sandpaper; A welding point to refit the lever ( Even in the middle of nowhere I found somebody with a welding machine). Be sure the wastegate is firmly closed and , although the volvo manual says to move it from time to time, do not do it!!!! You will never be sure it is well closed and the risk of overpressure is so small on a sailing boat!!! which should not have a turbo anyway.
I now get 3100 RPM, before cleanng the injectors, which I never did (shame on me) and am doing now.
Yves MARTIN d'AIGUEPERSE
S.V. BLUE MARINE