I came across this rather interesting document from the Royal Latin school (a Bucks grammar) which says what the average child is achieving there at the end of Year 7.
learning.royallatin.bucks.sch.uk/file.php/1/Student%20Progress/yr7_expected_progress_summer.pdf.
Confirmed my view that DD is better off at outstanding comp rather than a grammar, is achieving very well in Humanities but would be seriously overwhelmed in Maths!