Engine RPMs, bottom
One thing I'm curious about is what speed the engine is run at. If it is run at less than 80 percent of maximum continuous RPMs then that could be contributor. A really big factor is how clean the bottom is. Just after a bottom paint the boat really rips. As time goes on the top cruise speed at the same RPM falls off. I'd put bottom cleaning high on the list as a potential problem. After the above, I'd recommend checking the pitch of the prop. Does the transom turn black from soot? The Max-Prop folks (or any reputable prop shop) should be able to help with ideas how to check the prop without taking it off.
