critical level of lead in transmission fluid analysis

Well, Yes and No.
Cat does measure engine life by fuel consumed per engine. So, less HP, Less fuel consumed then supposed longer engine life.
Ah, But here is the rub;
The above rule is tended towards engines run at designed HP & RPM.

Just slowing down the RPM of a higher HP engine may save some fuel, it also keeps the engine from running at it optimal temperature and tune.
Now your not at designed HP.
Reduced RPM may reduce the head pressures, over fuel the piston, rings and liners.
Now, lots of low rpm operation and this engine, that designed to have boost pressures and not getting it.
Over-fueling and increased (accelerated) cylinder wear come to my mind.

There was a reason that 300HP was installed in this hull, It was not for drag racing nor for extended low RPM operation.
I would suggest the boat design requires around 225 to 260HP (16-1800 RPM), if less was needed, the lesser HP engines would be installed. You can only push a displacement hull so fast. But some extra ponies to push against wind and waves are welcome sometimes.

Yes, near 5600 hours is a concern for this engine.
But if maintained and ran well, who knows how long it could be before a real engine block event happens. Just hope not in a foreign port.