when would you go to 1.36 from 1.50 ? ....I assume you'd need enough "umph" to turn the taller gear .... There a rule of thumb at a certain HP you gear up ? Do the Transmissons have a gearing that would call for a different gear ?

General rule of thumb is if you start to need more than about 33" of pitch with the 1.5's. Seems even the 100+ mph setups are coming set up with 1.5's. Going to 1.36's without the power to pull it will just cost your boat in driveabilty and acceleration, and by spinning the propshaft and prop faster, will most likely result in poorer efficiency.