380 flybridge / 3126 cats

That could definitely account for it as could a dirty bottom, bent shaft, bad cutlass bearing or struts.

DID you have engine survey done??? If not I would do that first, 550 RPMS shy of WOT is A LOT. I would also try to do another seatrial, make sure all fuel filters are new, air filters are clean and see what rpms each engine makes by itself WOT (1 at a time), I would also measure what boost is at cruise and WOT as well as fuel pressure. If the current mechanic is not a CAT dealer mechanic I would get one. If the bottom was clean, I'd be very wary that there isn't something major going on with 1 of the engines or both. Honestly I wouldn't move forward with the deal at this point without verifying it's not something in the engines. You can't even cut the current props enough to get the rpm's you're looking for as they're so far off. So you're looking at buying new props. What was speed at WOT. What did the boat weigh? Is it within spec?

Running diesels that over loaded is not good at all. I smell problems cooking

Running diesels that only make 2300 RPMS when they're supposed to make 2850 RPMS WOT is very bad for them. In my mind a 550 rpm loss of what the motors should turn is HUGE. To me this tells me there is something wrong with the motors not making enough power due to a mechanical issue than it does props. I would imagine that these props origionally turned 2850 rpms when the boat is new. 550 rpms is a sign of a serious problem.

Diesel motors do not like being over loaded. 100 rpm raises flags. 500+ is huge. The pistons, crank, bearings are under much higher stress with less coolant flow on top of that. Ride your bike up a steep hill in to high a gear, that grinding and popping sound is what is happening in the motor. It also hints at poor maintenance from previous owner, why did they let it get that bad. Diesels are great powerplants if maintained properly

The motors have lived a very hard life if they only turn 2300 rpms WOT, no matter what rpms you run them at, they are being run in an overloaded condition.

You don't just lose 550 rpms. The original props, just don't become over propped without a legitament reason, such as boat weighing considerably more or engines no longer performing (making the same power as they did new), or an issue such as the bottom being dirty. 100 rpm's difference between rpm and WOT rpm's, given the age I could see, 550 rpms lost, is a GLARING red flag that you have a big issue somewhere. Are the props that are on the boat the factory prop specs when the boat was new?