The most apparent difference between grammar and non-grammar pupils within selective areas was a short-term positive effect for pupils at selective schools at the age of 11, regarding belief in their abilities, which the researchers called “academic concept”.

By age 14 this was no longer the case and there were few positive effects on wellbeing from attending a grammar school. Researchers concluded that policymakers who were looking to improve access to university, or “educational equity”, should consider reducing levels of academic selection.

However, on average, grammar school pupils were eight places higher up the ranking than a comparable child within a comprehensive area.Pupils from all types of schools had similar levels of vocabulary and motivation but children in selective schools had higher expectations of staying on and attending university.

The study, published today in the American Educational Research Journal and funded by the Nuffield Foundation.

It's not clear from your post whether they differentiate between Kent systems and Essex systems.