Try decarboning the engine first. Just be very careful not to overdo it as it can actually damage the rings and cylinders. What type of spark plugs are you using, brand and model? If it's not NGK, I recommend you switch to NGK's BUHW.
Does the engine has any misfire at WOT? Even infrequent misfire can hold back WOT speed. Infrequent or irregular misfire at WOT is usually an indication of fuel starvation at WOT settings. Meaning the fuel pump may not be delivering the proper amount of fuel at WOT speed. If you have the clear fuel filter installed between the fuel pump and the carbs inlet, this will easily tell you if the engine is starving of fuel at WOT speed as the clear fuel filter will be empty or less than 1/4 full.
Another cause of irregular misfire at WOT is timing or it could just be a trigger wire getting pinch at certain position of the trigger coil.
Does the boat track a straight line except for the counter torque from the prop which should have been compensated by the exhaust snout? In short, do you have to steadily compensate with the steering wheel for a straight tracking? if so, then adjust the exhaust snout to reduce drag.
There are a lot of factors that can affect WOT speed and most especially with load. I understand you were by yourself when you did the test. Could it be the boat is already waterlogged? You can tell by after getting it off the water and on the trailer, remove the drain plug and see if large amount of water comes out. If it does and continues to drip slowly for a long time (minutes), there is a good possibility it it waterlogged.