Johnson 9.9 vs 15 rpm question

I know the 76 through 93? models were basically the same motor with the exception of the carb, exhaust tube and a shim. I also realize the rpms of the 9.9 were set about 1000 less than the 15 in which the carb provided the extra ponies. My question is this, with these motors basically identical is there any danger to have a pure 9.9 and put a prop on it with a smaller pitch (I think you can get it as low as 6) to increase the RPMs into the 15 hp range? Has anyone actually tried to do that and what are the results?

I am not going to try this (I have a 15 hp carb installed), I am just asking for curiosity sake.

After the carb is flowing all the air it can, additional rpm actually decreases power. The torque falls off sharply. That's why the bigger carb and expansion chamber type exhaust; the greater flow capacity moves the torque peak up to a higher RPM. Now the extra rpm is moving more air/fuel...this is what makes more power.

This is why a too-small prop will slow a boat down even though the engine is running faster.