"I have a DS exactly as you describe yours - except that he will go to our local comprehensive because it gets better results than any of the 'highly regarded selective private schools' locally."
Unfortunately I can't say the same. The local comps here are ok but not great.
"He is 3+ years ahead in maths..... He does not help others with their maths because his (state primary) school teacher seems it as her job that she ensures he makes the maximum progress possible while in her class and she does not see that helping others is a useful use of his time. "
DS's teacher did it to keep him busy
. He would finish the 10 minutes maths challenge in 2 minutes for example. Rather than set him harder work she would simply get him to help others that were struggling.
"I therefore don't see a link between Kumon and being academically able, or between it and having a ferocious work ethic etc .... Your son might be exactly the same without Kumon. Mine might well have been the same with Kumon. The point I am making is that there is no causal link"
I am not suggesting a rigid link either :)
All I am saying is that it taught him to focus and to concentrate and to work to the clock.
At DS's school there was no homework at all. It wasn't particularly academic and as I mentioned above the teachers did nothing to develop my son's potential. If he had your school and /or your teacher then I would have been happy to chuck Kumon out the window and save myself £50 a month.
What I am saying is that Kumon suited my personal circumstances. I am not claiming that it automatically churns out high achievers :)