I agree. I think running 110 weight oil helps. Never had luck with Merc stuff.

One thing I`m certain of is if i gave you my boat for a day and told you to try to break the 4`s only getting on plane you`d be tired at the end of the day but the drives would be fine.
Before I got good props a 2 minute 5000rpm burnout was standard procedure

Can you say burnout , slippage ,not hooking up, not loading drive etc. just saying !

Running 950+ through a bravo xr. Rebuilt after about 100hrs. Gears worn out. I Don't baby it coming on plane just don't do anything stupid. Half trim at 60 and hammer it, excelleration is amazing. Haven't blown one yet,,,,,,,,, yet.

what size prop,# of blades and gear ratio?

I babied my fully upgraded imco SC drives behind procharger 500efis in the Baja for years. Rebuild drives every season around 30 hours. Became used to the boats speed and sick of driving like a grandma. So started driving like i was in a race truley enjoying owning a 100mph boat! harder and faster I ran her the longer the drives started to last. Crazy I know.

The science to me is max torque is max torque. Cruising speed with the boat dragging through the water WEARS the drive out. Most engines are making about peak torque at cruise speed. Getting on plane does not break a drive. Just plain using the drive prematurely wears it out. Simple.

Now getting air, especially small blips, will break a drive

Getting on plane, cruising, or driving fast will not make any noticable difference in how long a overpowered drive lasts.

I can one up CAB on the XR.

350hrs of 1250ft lbs heavy diesel torque into a 2007 1.26:1 XR spinning 5 blade of 35 or 37 pitch pushing 5300lbs. Could blow a 4-blade 32 bravo out of the water with ease.. Only known mod, IMCO output shaft. Stock shaft failed and was replaced before I purchased the boat. Besides a shifter lever snapping, no other issues. Opened the top a couple time to inspect, but never pulled the lower in 5yrs. Still on the water todayd.

That said, I did torque limit the power last year to about 850ft lbs below 2400rpm when I noticed the upper swivel pin showing wear. It only saw BIG water twice in it's lifetime with major air multiple times then, that being key in my book. Smooth water, 60 mile runs for lunch was a normal day.

5 blade hydromotive p5x 30p with 1,36 and -2 sportmaster

I'll agree 100 percent on the hp doesn't break drivetrain torque does. I truck pulled for years with 700hp all the way to 1100hp cummins. I never broke anything at redline. Only time I broke driveline parts other then bouncing was when the weight of te sled would suck the power down and I would get under the turbo in the peak torque range. Then you could really watch the parts fly. So I am kinda on the theory where cruising around in the peak torque range can be just as bad as topped out.

That's where your science is flawed. Max (Peak) torque can only happen when your throttle is wide open.

Cruising at the same rpm your motor makes peak torque at full throttle has nothing to do with each other, other than the rpm.

So, don't let that keep throwing you off.