[Sans Peur]

If it is using more fuel and still not gaining RPM's sounds like an air flow restriction. I read where you replaced the turbo. What does the turbo pressure do when you try and increase throttle? Have you inspected the air intake filter and adjoining hoses? On a diesel forklift I had a hose get weak and would collapse internally at higher rpm' s restricting flow.

Also check your oil pressure, the turbo is powered from that and a restriction could not allow it to spool up higher.

Next step would be to start looking at the exhaust side for any restrictions. Do you have straight pipes of mufflers? I have seen mufflers delaminate inside and cause a restriction.

Diesels have to be able to breath both inhale and exhale. If either is restricted even just a little it is going to cause problems.