11 is too young to be written off as not deserving of a good education.
That's true, but this is truer: nobody should be written off as not deserving of a good education. Which is what a grammar/secondary modern system does.
It's true that for the most exceptionally able, a comprehensive can't meet needs, any more than it can for those whose disabilities mean they can't access the curriculum in mainstream even with appropriate support. For that reason I think there should be a single grammar per county, which selects only kids likely to be genuinely gifted - not just very able. And those kids should be selected initially via the SATS, which should have a supposedly non-coachable element without a planned ceiling.. and any child found to have been coached would would forfeit any grammar place at once, and any coach would be fined in the same way someone cheating at sports would be. It's an unfair advantage in accessing tax-funded resources, and at the moment, kids who aren't coached have a massive disadvantage, which entrenches the existing class bias. Why should we all pay for that? Where's the public benefit?
A super-selective trying to serve only the startlingly able would need to have a flexible approach, within reason, to numbers, so interviews and even an ed psych assessment should be made for the borderline cases (many will be glaringly obvious) to see if the child is genuinely in need of a place, and not more than capable of thriving in a good comprehensive. It should be seen as a special school for kids with truly remarkable potential. Because they aren't catered for in the current state system, in many cases, and can end up totally disenchanted and disruptive. Plenty never achieve at their actual potential, and that's a loss to the country as well as to them. We have special schools for those who can't access the curriculum at one end - seems a shame not to at the other. (Also worth pointing out that some kids will have overlap - you can have disabilities in some areas and gifts in others, and the additional support would be even more valuable to those kids, perhaps). A school with a few hundred pupils per county - a truly super-selective - would not impact the funding or overall talent pool of comprehensives, and wouldn't label any kid who didn't attend as thick.
At the moment, grammars cater to the carefully coached, fairly bright upper-middle classes - those who can afford private school but don't have to, and those who can't but would like to. Endless data proves it. That's just a private school on state money. I don't see any benefit to the country as a whole, and in fact data tends to support the idea that they discourage social mobility, not the opposite. The free school meals statistics show how few poor children attend grammars. It's a tiny proportion.