Everything I was going to say has been mentioned already: the banding tests which ensures you knock out lots of kids even at the application stage; the insanely high proportion of high prior attainers (almost two thirds of the cohort vs 20-25% in an average comp), which means their results should be exceptional; the mediocre progress 8 (value added) score which has been underwhelming and mostly negative for years; the BS scholarship system (they are not alone in this in south london) and of course the hype around it, driven partly by the lottery system and the scholarships and the sense that you "win" or "earn" a place there.
There's also the school bus, which costs close to £1000 per year and ships kids in from across south east london (not sure about elsewhere): this is a great option, if you have a spare £1000 a year per child, many do not and again this will rule in some children and rule out others.
I know of children in years 9-11 whose parents are underwhelmed. I'm sure this is the case everywhere, of course. It's probably a perfectly fine comp with some great clubs and provision in arts etc, which you would expect in a school that size.
And yes progress 8 DOES matter, for high attaining kids as for everyone else. There are many great schools, grammars and comps, which do brilliantly by high attainers (and for comps - also middle, low, disadvantaged cohorts). It's simply not the case that a P8 score will be low because the kids are already doing well on entry. It's not the only thing that matters - but if you are going to fixate on the results it makes no sense to ignore the P8 score.