I had stock merc xr ujoints behind 1000 ft lbs tq and hp for 16 years/500 hours in my baja, still like new when inpulled drive off boat before selling it snd stuck stock spare drive on, kept them greased though. Ran that boat trimmed out too at boost quite often, fwiw

This sums it up right here. I have 650 ft lbs tq on my 540 and if I would have greased them with some good synthetic grease, the stock Merc ujoints would have lasted far more than 5 years.

For ujoints, pay Merc their inflated profit margin and save yourself the time and trouble.