bending is the issue, as Im sure you are aware. If the rudder stock is strong enough to survive the potential bending load, torque is well covered.
With a skeg- or keel-hung rudder where the stock is supported top and bottom, torque is the major factor in designing the stock -- although a skeg must be designed to support the bending loads generated by the rudder.
The old rule of thumb in the days of ink on vellum was to assume the rudder square on at 1.5 times hull speed and add a safety factor to that. That made rudders on boats that surfed easily marginal to say the least (load increases with square of speed). I imagine the computer people have some algorithms to do a better job -- especially given the kinds of speeds we see with Open Class monohulls, etc.
What we see today is boats with small keels. The loss in lateral resistance is compensated for with big rudders. Big rudder, more area, greater potential load.
I can see where you, DDW, are willing to put a few extra bucks into making sure the rudder is more than up to the job. Unfortunately, the accountants who run businesses today, and whose bums are not going to be sitting in wet cockpits in the Gulf Stream, might not take the same view.
But . . . as someone said somewhere in this thread . . . we don't seem to hear about too many rudder failures (perhaps because we're not told about them).