LaQueen - I was talking about comps, really. In a Grammar school, it is highly likely that a DC will shine across the board - they've passed an exam to get in, after all, but we have students who are A* candidates for English, but B grade for Maths or Science, and so set accordingly. I have G&T students in my own subject who are only hitting Cs in others.
Having taught in what was the top comprehensive in the area at the time, in a non-grammar county, I have seen the more more more approach work perfectly well in a non-selective school, where all the students in the top sets were getting A* and A grades. I don't think grammars are necessary to ensure that high achieving kids do well - if they were, then a) every county would have them and b) counties without grammars would get poor results across the board.