I agree with aliportico. If the school is right for your child, and most importantly they are happy, they should do well.
If they go somewhere, where they aren't happy, this will affect their work.
Being in top sets, in a good/outstanding school with parents that are supportive is just as good as being in a grammar, IMHO.
All schools have their good and not so good points. At the end of it, they all (hopefully) end up in universities, which take from all secondary schools, regardless of type. My niece, having missed the 11+ by a few points, went to my local 'Good' school and is happily in her 2nd year of Finance at Nottingham now.
And as aliportico advises, taking a look at the dept of ed performance tables, helps. Especially at the VA, this is where the closest information of how the school really performs, lies.