Gear case ratio relationship to prop efficiency

Since it is possible to alter the gearcase ratio on a 2.5 Merc with different lower ends, is it preferable to maintain a certain maximum prop rpm to prevent prop blowout?

My question relates to prop efficiency as prop rpm goes up since I assume once you hit a certain water speed over the prop surfaces you will hit a wall where an increase in prop rpm causes cavitation at the prop leading edges?

I would like to discuss the effects in a submerged application as opposed to a surfacing application since the results would be different for each type prop.

I realize most of this is theoretical and fluid dynamics related but obviously the manufacturer would recommend a gear ratio/gear case type for specific applications using hull speed and boat weight/HP rating to make a recommendation.

The reason I ask is with a steeper ratio I could turn a shallow angle pitch prop at higher prop rpm than say a shorter ratio with a steeper pitch. Since there are so many related factors to consider is there a "laymans rule" for gearcase ratio selection?

Thanks.