Hi OP, my DD is in Y7 and at Newstead Wood, same as another poster on this thread. I think it counts as super-selective, and I have been very very concerned about whether it was the right choice for her for the reasons you outline OP.
DD really only wanted to try for a grammar as her very academic older brother is at St Olave’s. It’s a great fit for him, but DD is much harder on herself, has to ‘graft’ a bit more than DS, and we have a very very good, recently refurbished girls’ comp a 20 min walk away, rather than an hour’s commute away...
Long story short: she loves it. I think, to be honest, she would have loved the comp too, but she has made lovely friends, joined in ‘the life of the school’ and is not just coping academically but getting great reports.
I worried it would be all full of ‘alpha’ girls, and my quieter child might struggle, but she has found some like-minded friends, and seems happy enough not to always be top of the class/ up at assembly/ representing the school/ county/ national team in umpteen sports 😉.
If you have any questions, do feel free to pm me, and good luck with whatever you decide to do.