The fact the school has opened its doors to girls should tell you a bit! i.e. they weren't quite as oversubscribed as one may have been lead to believe.
Also, bear in mind, the DC in the Prep no longer have to sit the exam (though are allegedly 'weeded' out a bit prior to the day by which they either have had to commit to the upper or to another more suitable school). Also, someone I know very well (which is where this unbiased info comes from!) was actually a bit pissed off that his DC had worked hard towards his upper entrance exam (was in the prep, the last year that sat it) yet every DC in the class passed. He felt it was an exercise in snobbery, tbh, that it should have sorted some sheep from goats! His second DC- well, he was called to the upper to be toldhe needed to be aware that this DC wouldn't necessarily have passed the entrance exam as one of his subjects was quite poor -but whose entrance to the upper was automatic as he'd been at the prep. He recognised this as a 'don't blame us if the DC doesn't pass reams of GCSEs or iGCSEs or whatever!)
So, I know 2 DCs personally in the upper. They are pleasant, normal DC. They are not particularly bright and, dare I say, not appreciably doing better or learning more than my DCs in a good state comp. This is not to 'dis' that school. I am sure you get what you pay for- they have good facilities and class sizes of about 22. And no disruption, of course as one reason for the 'entrance exam' is to weed out 'trouble'!) My DC will have to wait to Y11 to get classes of 18.
But my friend feels the school is sitting on old laurels and shouldn't really sit quite so firmly on them these days as the entrance base has broadened, which is not to say there aren't some very clever DC there- of course there are- but it might not be the academic hot-house it once was.
Good luck, anyway!