Why is it not a good example to use?
How many EAL students are there? How many ethnic minority students? How many students arrived in year 10 from a non English speaking country? Out of those immigrant children how many attended grammar schools? How many SEN children are in mainstream?
Ideally, you would need 2 LEAs with similar KS2 results, similar parental backgrounds, school funding etc - but one would be wholly selective and other non selective.
Back in the 1980s Burnley had grammar schools, selection at 14 rather than 11 though.
All students went to the comp and at 14 the grammar schools creamed of the top sets at all the 'comprehensives'.
Except for the RC schools. The RC students' grammar was in Blackburn and they had gone totally comp so the RC high schools became comps.
The top sets of the RC schools got the same sort of results as the grammars, the lower sets at the RC schools got similar results to the high schools.
Obviously not an ideal comparison but probably as good as you can get.