When a motor is over propped it causes greater work and heat at any rpm than the designers planed for at that rpm. There is a safety cushion built in but it is there for a reason.
I believe we just went through this....about five times. What "designers" are you referring to? You seem to be thinking that the engine was designed for a specific boat. It's not. The maximum rated power curve developed by the engine OEM is there to cover installation in a range of boats. A small overprop in your boat might be an underprop in another make/model....and the cooling system has been tested to accommodate that range of installations. Again, the "designers" (of the engine) planned for different operating scenarios/installations. They provide the boat manufacturer with installation information to insure the installation doesn't screw up what the engine guys already addressed. If the installation is wrong...the engine folks won't warranty the engine. Excess capacity across the rpm range isn't a "safety cushion" as you call it. It's power capacity that's been tested as safe to use. The term "cushion" is often used in the context of "cruise prop with derated engine" as a way to convey that virtually all engine installations have approved capacity far in excess of prop demands. I urge you to get some engine and prop charts and think about what they depict.