No doubt you've seen the latest secondary league tables, which cover both state and private schools. (http://www.bbc.co.uk/news/education-16729387
All seven of Gloucestershire's grammar schools are in the top 200. This is out of almost 4,000 secondary schools in England. So they are all doing a brilliant job & I hope this provides reassurance to anyone whose DC does not get their top choice GS in March. All our GSs are great.
The county I grew up in doesn't have a single school in the top 200, not even the well-regarded private schools.
There's loads of interesting information if you are prepared to drill down into the individual schools' performances. They split the children into low / medium /higher attainers on entry, (I think this refers to whether they were Level 3, 4 or 5) and then show what % then reach their expected performance at GCSE. There's also some quite shocking figures on the discrepancies in outcome between "disadvantaged" children (those on free school meals or in care) as compared to the others within each school. This information is suppressed for most of the grammar schools, because they have so few in that category it would identify individual children, although no doubt the govt can see it. There's wildly different outcomes within the comprehensives though.