Just because the students are academically selected, that doesn't mean we don't have diversity. In fact, I'd argue that we'd have MORE diversity simply because we have students from a wider range of socio-economic backgrounds, ie. not the 99% "rich" kids as in the elite private schools.
On your second point, the difference between a 90/1 median and a 94/5/6 median is huge and a lot of kids would take that extra academic acheivement over 6 tennis courts, 5 ovals and whatnot tbh.
As for the top ends, thats probably true, however MHS and Mac.Rob can't put in the "effort" to "nurture" our best kids like what the elite private schools are known to do due to our lack of funds. Furthermore, thats not why our schools were made selective in the first place. We have a completely different ethos.
@argonaut, having never been to an elite private school, thats a difficult question. But I doubt I would have done better at MGS or Scotch.