It's not all about best exam results for all kids.
Plenty of kids (and it's teachers who have told me this) are just not cut out for academia. Some of them would benefit from learning that you don't wear sportswear to job interviews and what the safety signs on a construction site mean. Some of them are better off spending their time learning the trades they'll be using when they leave school.
If you restrict your measurement to grades achieved in subjects that have no relevance to many people's life outcomes then grammar schools probably do lower the grades for those at comps. I've never seen a longer term study that shows whole-life outcomes, but I suspect the results will be very different.
I think these are very good points.