Efteling
There is every chance that, should the proposal to create a few more Grammar Schools in comprehensive area go ahead, that most of the remaining comps will deteriorate, and most of the non- pupil premium places for the new Grammar School in the area will go to children of people like mine.... who currently go private. So a lot of the working and middle class Tories/UKIP supporters kids will not go to the Grammar they agitated for and end up at worse schools than they are already.
Some places will go to a handful of on-paper poor kids (but only really those with parents with the social capital to jump though the hoops). The kids who will really lose out are the averagely bright, the ones who are only good at maths or english, the late developers, the kids in care, those who move around a lot, child carers, those with SEN, plus all those who end up in the even less funded, less favoured other schools.
Historically, in the 1960s secondary moderns were funded significantly less per pupils than Grammars and there is every chance this will be repeated.
burningourmoney.blogspot.co.uk/2007/05/real-disgrace-of-secondary-moderns.html
If we are trading anecdotes, in my comp, there was a lad taken into care at 9 who started year 7 (then 1st year) in the 4th stream (CSE standard) and was up to top stream by 14- he got 10A levels.
My school had previously been a secondary modern, no exams were taken and it had disproportionate more girls (as there were less grammar school places for girls than boys). If it had been earlier when this boy went to school, he would not have gone to Grammar and left school with no qualifications. By going to a comp, he did significantly better than under the system you advocate for.
I can see for a handful of very high performing pupils like my son, a super selective Grammar School education would be helpful. But he is Maths Olympiad/grade 8/9s in everything standard and most of the Grammar school supporters kids are not (not a boast just context). The Grammar schools were largely abolished because too many parents were not happy about their kids (who they felt were too clever for a secondary modern education) failing the 11 plus and it became politically difficult to justify the divide.
I do think that many comps could do more to stretch the very able (like facilitating them to start university courses on line ) but this is a wasteful and retrograde step.