I think superselective grammars and very selective private schools operate on a similar basis. They select the very brightest kids and then the curriculum is tailored to those kids and the pace is much faster in pure academics and then there is a ton of extracurricular and societies to support their wider interests. Top maths sets everywhere do not spend that much time teaching and spend more time on greater depth questions/UKMT type questions. It is a given the students have already grasped a lot of the material. Parents who overtutor are deluded in thinking their just upper average kid is going to necessarily do better in a school like this. They would often be better off in a school that really handholds and teaches to the test.
The downside is that those kids in superselectives do not realise how bright they really are as going there they are “normal”. Feeling normal is nice for a teen though.
Equally, for equally bright kids going to comps who are in the minority there (which is a fact for most comps 11-16) they may feel overly special and bright and once they get to eg Oxbridge they can tank and their confidence takes a knocking when they are suddenly surrounded by lots of other very bright kids. This is when the superselective lot have a confidence advantage.
At this juncture, we should also highlight that a lot of the most academic private schools like Westminster and Eton have full bursary programmes for truly gifted children as well, and often you can get 105-110% off school fees to cover uniform and trips as well. These places are available to all boys across the country. And here we should also highlight and question why there are not as many girls places!
For some reason, properly gifted children is a very emotive subject in the UK. Of course these children have a form of special educational need. And they deserve to be challenged. Many are not, up and down the country and that is a fact.