Most people think bigger is better as far as efficiency and diameter of the prop. Then there's the question of is the reduction gear deep enough for a big prop.
But propeller loading is perhaps more important than most else. However there's no doubt a larger slower turning propeller is generally more efficient.
3 to 5% slippage is really small. Must have been a very unusual setup.
the "prop curve" FF is talking about is from horizontal = prop speed (higher to the right) and vertical = power applied. I almost said thrust developed. Max engine speed is to the right. Max power is low or toward the bottom. A 100rpm increase on the left side increases the power applied but not much. A 100rpm increase on the far right increases power a lot. Perhaps four times as much as on the left.
This curve is not accurate but fairly close. It's for a typical hull and a specific engine usually that of the graph and engine maker to aid the customer or yard where they are setting up the boat or refitting. The engine manufacturer wouldn't be inclined to make a graph for a specific boat unless hundreds of their engines were to be installed in the same boat like a NT 32. Or they'd have to make thousands of graphs.
The graph is meant to get real close re prop dia and pitch but there are other differences as well like boat weight and prop type or style.
Al go to an engine manufacturers web site and see if you can find a theoretical prop curve.
North Western Washington State USA