Dunno. Depends on the child I think and how much their self esteem hangs on academic competition. My 3 I think get self esteem from external sources (musical).
DD1 went from in the top 2 at primary, to grammar with her best mate ( the other of the top 2!) and they just continued to be at the top, fighting for the class prize etc! Nothing changed LOL
DS we rarely knew how he was doing (he didn't really, well care isn't the right word, but comparison with peers, except in rugby didn't bother him). He went from top 2 or 3 at primary to.... dunno from year 7-11 to top few in year 12 where he's started benchmarking himself again. But he's been happy which is what matters.
DD2 know's exactly where she is I'm sure and it's again fighting for top but she plays it cool LOL
I know other kids that were peers of DD1 and DS who went along the comp route and were much much happier to be at the top all the time there than they would being middling /bottom at grammar. Parents " devastated" etc when they didn't get in but they have done OK and rejoined grammar at 6th form with a better level of self esteem and coped well/gone to uni etc
I also know some kids who have had a miserable school career at grammar, hanging on by their finger nails. one mate of DD1 especially. She is now studying medicine in Bulgaria because she didn't get a UK place on 2 rounds of applications. I do hope it works out for her, but I don't know if she's happy (and what quality a Bulgarian medical degree from a non central university will be ??