"I'm not sure that value added scores are as important for grammar schools that regularly get very high results as they are for comprehensives. "
Educationally, that is an appalling thing to say, as surely what is MOST important in terms of the education of every child is that they make the maximum PROGRESS that they are capable of.
If value add is low, then the children are not making the progress they are capable of. Especially in the case of very able children, who have shown a great capacity for making great progress, a lack of progress is really worrying.
[There is, to be precise and fair, an issue with value add for superselectives, because to make even expected progress from their starting points, a higher than A* grade might be needed. But that is not the case for the vast maority of grammar schools]
My proposal, by the way, for identifying the children who cannot be effectively educated in a comprehensive school due to their absolutely exceptional ability would be exactly the same as for identifying children for special school at the moment - a range of tests and personal reports adminstered by an ed psych.