Injectors usually last longer than 138,000 miles, but with substantial kerosene use I could imagine them being prematurely worn out. Unfortunately, I could say the same for the injector pump. If the car is making ten psi of boost, the turbo and related controls are working more or less normally. If it's blowing black smoke, that points to it getting plenty of fuel. However, if the injectors are popping off at too low of a pressure or have a lousy spray pattern, that would produce poor combustion efficiency and the observed phenomenon of low power with accompanying smoke. I would suggest pulling the injectors and having them tested at a shop. Some shops will even do this for free.

The valve adjustment is a regular maintenance item and should be done if it has been over 15,000 miles since the last adjustment.

The only other things that come to mind are EGR stuck open and late injection timing. The EGR can be eliminated and there's really no reason to retain it unless you have stringent emissions inspections to deal with. Injection timing shouldn't have changed in the stated mileage, but I suppose it's possible the timing chain could have somehow jumped a tooth at some point.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.