Bertrand top 2% nationally is a typical definition of giftedness. Its two standard deviations from average (circa 130 IQ). Its been studied repeatedly and proven that at that threshold and above children benefit academically from specialist provision. These kids learn at a much faster pace than average and its very difficult to stretch them in mainstream classes leading to, for some, severe academic boredom and underachievement. Some kids like coasting though so every child is different.
In the US at least, the optimal time for testing is seen to be between 6 and 8 years old though these days multiple forms of identification are preferred to only an IQ test which can have a number of biases. Early testing is less reliable in younger children.
Alexa most grammar schools take the top 20%. Only a handful of super selectives (aka Tiffin et al) take the equivalent of the top 2%. In a heavy grammar area like Kent, hiving off the top 20% into a separate school can be damaging for the children left in secondary moderns. All of that is to say, its not the same thing at all and the general analysis for grammar schools doesn't apply to super selectives.
My local comp here in London has a small gifted and talented stream within it so there are multiple ways one can make the provision. When I was in primary school in the US, the gifted program was within the mainstream state schools as a pull out program. It was a multi-age class of circa 20 kids.
I agree that drive, emotional intelligence (including social skills) and resilience are all key to leading a happy successful life. This is just about the right educational environment for children's different learning needs.
I find it interesting something so basic could be interpreted as people saying the children / parents think everyone else is beneath them! No one ever says that about children going to specialist sports or music schools. I think its a societal hang-up people have about intelligence which is sad...