Andy, I think you have what is a pretty good theory. But, it would likely only play out when the engines reached many thousands of hours, way more than a recreational boat is ever going to get to. These engines are used in commercial and military service which can put on more hours in a year than one of our boats will do over its life.I'm buying this boat for usable space and lazy cruising, frequently running at displacement speeds so I'm not concerned with having a particularly fast boat. I'm still trying to figure this out but if a 480 CE were run on the power curve of a 450 diamond bringing usable horsepower more in the range of the QSB, wouldn't the 5.9 displacement of the QSB be working harder than the 8.3 displacement of the 480 CE? Working harder equals more heat which typically results in shorter engine life. I guess when I asked if the boat with the QSB was underpowered it was more question of is the engine being overworked.
You might want to check out https://boatdiesel.com/ or the forum at https://www.sbmar.com/community/. There are real diesel geeks at both who would probably have an opinion about this issue.