[B2C2]

None of these replies mentions examining the air induction path to the engine. If there is blockage there the air delivery will be at reduced pressure to the turbo. If this is large enough it will prevent the turbo from developing full inlet pressure. A clogged inlet path or airfilter could cause this. During the bad fire seasons we had on the west coast the air filter on my turbo was clogged significantly with smoke particles. I did not see a turbo pressure issue but didn't take it to high altitude much either.

As a side note I had a problem where I could not get to full inlet pressure on my front engine a few years ago. It turned out the magnet that holds the emergency air inlet closed had fallen off and damaged the impeller on the compressor side. For the posts where the turbo was replaced this obviously isn't the issue but if its a new observation make sure to get a look at the impeller to make sure its not damaged.