A most interesting discussion. I have not had much experience of Registered ESP as yet - only a handful, all at the same ground. I would have thought that if the course designers aimed at a winning score by an AAA shot on a good day of mid nineties, with a fairly even spread downwards through the classes then that's fair enough.
There's little point in making a course so difficult that the winning score is in the low eighties, other than by accident (weather) - I for one would become a tad dispirited; the attraction of doing another one would be less. Major championships that don't count towards one's classification could be as tough as the host wishes, in fact they should be tough courses, after all it's better if the top competitors can be separated a bit.
Others have said that there are "straw balers" to cater for the less accomplished. Of course theirs may be a different customer base, attracting the proficient Sporting shooter all the way to the occasional pigeon shooter and wildfowler along for a weekend bit of fun. That's the way two of our local shoots do it, they want a winning score close to a straight, but they want the occasional shooter to have fun too. They cater for all customers, and the two that I attend round here do a pretty good job. Perhaps they should set the course so that it doesn't need an AAA shot to straight it but it would be a bit of a stretch for a C class.
Like so many things, so many activities, it's all about encouraging those that may follow, not putting barriers in the way to their enjoyment.
On the subject of classification, it's a bit of a conundrum that there are those that (I read here) only do the easier shoots in order to gain a more prestigious set of initials, and there are those who apparently artificially depress their scores and classifications in order to stay in a lower class so they can win more. I really don't know what anyone could do about that one!