I think we all agree that superselectives are a different issue.
I don't agree that superselectives are a different issue. People keep saying that superselectives don't impact on the comprehensives but this is not always true - you have to draw children from a very large range of schools to have no effects on surrounding comprehensives. This is possible in the London area, but not in most of the rest of the country. Some schools which people would call superselectives do have all the negative effects of grammars.
Look at the Dorset schools in Bournemouth/Poole. These are superselectives in the sense that they do not have fixed catchments, and children can travel from a long way away to attend. Children who travel from 10+ miles to attend are pulled from their local comprehensives, but rather few children are lost to these comprehensives (it's too expensive and too far to travel to the grammar), so the grammars indeed have little effect on them.
But now look at the secondary schools closer to the grammars. There are quite a lot of them, so at first sight you would say that they are unaffected by the grammars. This is emphatically not true for schools within a few miles of the grammars - they have lost almost all their high achieving, affluent children and are most definitely secondary moderns.
Then there are a lot more schools 4-6 miles from the grammars. Many children from these areas don't take the grammar exam, so the comps keep some high achievers. However, they still lose some to the grammar and have less very high achievers than they should, which affects the top sets and years 12/13 particularly.
Apart from the grammar schools, almost all of these state secondaries appear on Bristol University's list for contextual offers i.e. their A level results are below national averages.