I don't believe that if you put a child in a poorly performing comprehensive school that requires improvement, that they would come out with the exact same A level grades that they would if they had attended one of the top private/grammar schools. (Not saying that A levels are the be all and end all, that's a separate discussion).
I hear all the time that if your child is naturally bright, they will do well wherever they go. If that is the case, then why the competition to get children into the best performing schools?