It may well be true that people who have independent schools as an option, prepare less rigorously for the exams than their counterparts who have the option of a state grammar or a disappointing state Comp. clearly the incentive to prepare is stronger.
I would suggest that the numbers who get into Tiffin who are not bright, but heavily tutored will be few these days. Tutoring can take you so far....but not all the way, when competition is so fierce. Simple, innate ability will take a number all the way,nwithout any tutoring or preparation, but most who get in will be both very clever and also prepared - not necessarily from infants, but perhaps with a good year of focused prep from a parent or tutor.
Those Independnet schools are nowhere near as difficult to get into. It's a numbers game and quite simply when 1800 apply with a 10:1 ratio, you will get better candidates than when the ratio is much lower and many have been excluded because they never could pay anyway.
I think that those who really want their children educated with the extremely clever, pick Tiffin. The local independents are very good and deliver great and often similar results to Tiffin from a wider range of ability.
People who can pay do have more choices. It gives them the choice to prepare to a lesser extent because the completion is much less fierce. It gives them the choice to choose a school which is less competitive to get into and to say that the state grammar was never their first choice anyway, just a practice run, and it gives them the choice to enjoy the perhaps less intense experience and benefits of better funding too.
In the end, lots will have offers from KGS and the other independents who get turned down by Tiffin. There won't be many who are the other way round. This shows it is harder to get into Tiffin. Of course, many don't apply to both.