you should double check bottom and prop condition, these must be clean for optimum performance.
if the boat is capable of the performance you are seeking, I have no knowledge on that score, and you are trying to get 2600 rpm at wot underway then you will need to start with around 3000 rpm at high idle (wot in neutral).
check rpm with a proper tach, not the dials on the panel, then set high idle by adjusting the stop screw on the speed control lever.
strongly advise fitting boost gauges, even if only temporary (if you dont want the panel work).
remove air filters and sea trial after all the above and record results.
if you get continuous black smoke, not just a few wisps but plenty of it, it could be turbos problem, full load boost pressure should be around 20 psi if I recall correctly for this engine.
if boost pressure is 20 or above it isnt the turbos and then either the boat is too heavy for the prop / transmission fitted...or the engines are worn out !!
you can have the latter confirmed by blow by testing.
btw, if you intend to run the boat at that speed apart from using a lot of fuel you will soon have worn out engines, if they arent already.