the net effect of grammar schools is to disadvantage poor children and help the rich but that may be an artefact of the few counties where grammars remain in any number being affluent ones? As well as requiring certain test scores, as far as I know most grammars also have a geographical component to their admissions, so being able to buy a house in the right area definitely helps. The truly rich send their kids to independents anyway...politicians excepted.
As regards the FT data, a large percentage of students get A/A at GCSE nowadays (many more than when I took GCSEs back in the 80s) so it is difficult for grammars to show much 'value added' in their GCSE results e.g. you can't distinguish a student with 80% (A) from one with 89% (A) and likewise for A. A comparison of sixth form data and university admissions would be more enlightening. I haven't looked at the site yet. Maybe it does look at that, although I doubt it because then you get into the complication of different qualifications and so on.
I expect a detailed analysis would also show more grammars offering I-GCSE, single sciences etc, so there may be some apples and oranges comparison going on. In other words, the results & value added may be similar but the subjects and therefore progression to what the Russell Group describe as 'facilitating subjects' at A level may be different.
In the light of the discussion this week in the press about school standards I'd be really interested to see research into how students in grammar counties actually feel about their education e.g. do the more gifted or academic feel more encouraged to achieve (or not) in a grammar, do they get more chance to make friends with similar interests (or not), do they get the right amount of pressure, too much, too little...I would like to know that as well as exam statistics.