Treacle, if it's a full grammar area, is it one where everyone takes the exam or is it an opt in system?
What sort of percentage go to grammar? If opt in, do most opt in or not?
Sometimes sec moderns are called upper schools, because sec modern has negative connotations for some, from the days when the non-Grammars didn't offer an academic curriculum. However, in a fully 11+ area the non-Grammars simply cannot be Comps - the top whatever% is missing from them.
It is this issue that means people are often keen to avoid them, because they think the lack of the top set children makes a real difference. It can also make a difference to the level of parental support for the school.
What evidence seems to suggest is that sec moderns(non grammars) in full grammar areas tend to do best when they are in affluent areas. Those in less affluent areas don't do so well.
Again, as well as ability being important, a crucial indicator of the success of a school is the parent body and the level of support it gives. It isn't always the case, but often there is a strong correlation between this and parental affluence.
The real pity of all this is that it is the childrenninnthenless affluent areas who lose out. Yes, the clever ones get to grammar (although in smaller no.s than their affluent peers due to less preparation or tutoring) but the ones who don't get to grammar end up in schools that lack a decent number of the clever children and their parents, who are often affluent and interested and who make a real difference to the school. For them, it is worse than being in a Comp where there is more likely to be a fuller range. Yes, these are generalisations but are broadly true and actually show again how the full grammar system disadvantages those from less affluent backgrounds.