Not yet , I have question first. 
Is it true that most of the failures with stern drives are caused by shock loading the drive rather than actual power.
All "fracture failures" are from shock loads, most of the failures "real world" are either from excessive heat and or bad maintenance or shear horsepower causing case distortion which leads to bad gear mesh and bearing preload issues.
I have seen several different scenerios with the same outcome, blown up upper gearcases (rarely lowers), synthetic oil (I am an Amsoil dealer so that's what I recommend

) makes a huge difference in the operating temperature making things live longer, not to mention lowering the friction factor and increasing the horsepower output to the shaft.
Shock load failures are most common in offshore applicatons where the drive is put under an enourmous amount of strain with 1500+ Hp engines and constantly leaving the water at over 100 MPH, if you enter the water under power, big things happen, (ever watched the throttle man in an offshore boat?).
Most lakes (with some exeptions) don't get a large enough chop to have this issue, but still happens.
I'm not sure that totally answered your question...Hope it did.
GT