This thread has gone in multiple massive circles.
The only questions I am still interested in have to do with just how much do the high ability kids "underachieve" in the non-grammar system? I want specific hard numbers, like "0.3 grades less for average GCSE" or somesuch. & how much does that (eg) 0.3 matter, how much effect does it have on future life. I can't say that the comp system has failed my high-achiver DC at all.
Secondly: what is the function of the state system? Is it to provide minimal skills, (2)to cater for everyone regardless of ability regardless of cost (cost to state or cost to others in the system), or to provide equity of educational opportunity? Reason2 is the only one in favour of G-schools.
Third: NG said ages ago that Getting into Grammar schools is a huge emotional thing for many; they take pride in having got in, which is why G-schools have emotional support. Is that an appalling sign of selfishness, historical blinkers & belief in total BS about social mobility, elitism or what? I chatted with someone who is fiercely against G-Schools because he failed the 11+ & was a hyper-overachiever in the comp system, he feels cheated.... so why don't the anti-emotions predominate, if only a minority went to Grammars?