ACtually, what I then found were that when you look at the 5GCSEs (inc Maths and English) of all the comps in my area (south London), Lambeth Academy, Kingsdale, Dunraven, and the rest, they were all pretty much the same - all above 95% - and comparable to the partially selective Graveney (96%), so really only a handfull at most fail to get those 5 in each of the higher sets of those schools.
But of course a child getting 5 Cs is in that %age as is a child getting 5A* - and so looking at the percentage is not really enough. You also need to look at the
"average point score" (which seems to take the best 8 GCCEs of the higher set). Then you do get a stark difference: that figure for Graveney is 409, whilst for Lambeth Academy it is 375, for Kingsdale it is 393, and for Dunravern it is 385.
Not sure what this proves though. Except that perhaps only for my area it is not the case that the top set in a BSC get the same results in a selective school (one can assume that the "high" achievers in Graveney were all there through the selective entry route). But is the difference enough to warrant all the stress and hassle and tutoring to get into something like this. (For Tiffin, however, the average point score is a mind-blowing 440!)