seeker we fall out from time to time but on this occasion I shall grace you with complete agreement! 
As I said earlier, (my remarks re parents prepare to bash down school doors with battering rams to get their DC in), what the WLFS (and my DCs' school) benefits from is on side, committed parents.
swanthing- what do you mean by 'snobbish'? Do you mean my observation that sadly, in more deprived areas, you're more likely to find not just lower innate ability in the lower sets (which you'd find in a 'good' MC comp, too), you're also going to find poorly socialised, badly behaved DC? Who, if they'd come from 'better' backgrounds, might be in higher sets? But, as it is, merely trash the learning of the well-behaved but less able?
As for English- well, my DCs' school doesn't 'set' BUT it only allows DC who are likely to pass to take Eng Lit GCSE so in that way, they could be said to be 'setting'.
This remark:"But if he was in the third set, and he may yet drop down to it, I would feel that his chances of expanding his ideas would be more limited" demonstrates that you do not understand setting. If you trust the school- and your remarks indicate you do- and your DC is placed in a less academic set, this is because he has been assessed as not being able to access the speed and/or complexity of the work of the 'higher' set. They have placed him in a set appropriate to his ability- which of course might be the ability of a DC who has already reached the limits of their own intellect.
And finally, your DC is at an selective school. You yourself have told us it is a RC school (why?). It is endlessly shown that similar religious schools do better than secular ones. The speculation as to why is, imo, wrongly directed. To me, it's obvious that DC in religious affiliated schools do better because, pun intended, they're all 'singing from the same hymn sheet'. There is social cohesion. There will be non religious parents who have basically lied through their teeth, and been to Mass every week for years, and done the altar flowers for months- to get their DC into that school. They're committed. Their DC will do well.