Next on the list of factors is pressure. When you increase the surrounding pressure, you can usually dissolve more gases in the liquid. Think about your soda can. It is able to keep the fizz inside, because the contents of the can are under higher pressure.
Assuming a prop, it depends on how you are propped but with a redline of 6000 rpm, most outboards perform best with a cruise in the 4000-4400 rpm range. If you want top fuel economy, get a floscan or such and see what is the most efficient for your engine and prop.
The brackets can add 1 inch to the overall mounting height of the outboard. The standard shaft length for most outboard engines less than 125 horsepower is 20 inches. MarinePartsMan.com also says that V6 bass-boat engines have a 20-inch shaft. Outboards designed for saltwater applications usually have longer shafts.