I realise that "scraping along the bottom" sounds unkind, but I am convinced it is an issue.
One poster on a recent 7+ thread suggested her DS was doing 5 hours preparation a day. Say this child gets a place, but towards the bottom of the cohort. And has a coach journey of an hour each day.
The reason these schools get great results is because they move at a cracking pace. So lots of vocab to learn, and expectation that essays are researched and argued etc. Easy, indeed great fun, for those at the top, but for those at the bottom of the class homework will take longer and can start being a burden. I have not been through it, but understand that linked prep schools will flag up when progression to the senior school is not certain. So the poor child will be under pressure to perform week in and week out. Yes self esteem can be gained from other things like sport or music, but not if you are anxious or exhausted or spending your time on a coach. It also depends on the child. Mine was not particuarly competitive and enjoys learning and clever friends (he fits cake's stereotype quite well) so would not have minded where he was in class as long as he could keep up. This is not always true, and it is quite common in London for different DC to be at different schools depending on their personality.
It could be a decade long grind, and some will simply lose interest in academics, others get ground down. Sitting comfortably in the middle of another of London's very good schools can be a better option. Top University courses have international reputations to maintain and it will get harder not easier. For most a school like Westminster, SPS and KCW should nuture a love of learning, which is a huge asset at the next stage, but this won't happen if a child is fully stretched right from the get-go. And yes, there is tutoring. Not much amongst DS' friends though in GCSE year they discovered one boy could not join them on Saturday afternoons as he was being reguarly tutored in four subjects.
School will do a lot to discourage boys from competing against each other. However they can't stop it entirely. We were put off taking up DS' SPS offer when a Colet mother with a son the same age as mine, described another boy as "not very bright". Her's in contrast was in the top three in the year group, and she was able to list class placings for all of the top set. Part of the problem seems to be that some parents are used to educational systems where class ranking is very important, and so the British approach can be quite frustrating. (And can lead to awkward conversations where another mother quizzes you on your child's scores so she can determine how well hers is doing...) Also, I belatedly realised, the British tend to downplay their children's achievements. For a long time both DD and I were quite intimidated by some of the brilliant girls she came across. Only now, with GCSE and AS results under her belt, she realises that though her school was not considered as academic, she was probably just as able.
Its a long game. There are lots of useful things to do aged 7+. Learning Latin is not a high priority. Any school needs to be right for the child. Children learn best if they are happy.