There is little difference academically between school ranked 5 and 15.
However, one should think twice for the reasons, for example, choose an offer from school at rank 50 over a school ranked 5. There are many factors to consider without a doubt. Placing the child to the right bucket of school according to his academics ability is as important as other factors.
The fact is that nearly all the selective school use academic ability as their main selection criteria. The higher ranking, the higher level of popularity and higher number of applicants. It feel a bit silly to say such parents conscious endeavour is pointless.
As per value added, in state sector there is progress 8 score, but again it is usually considered together with attainment 8 score (derived from Gcse results). When choosing the right school. There are benifits, as in fact exercise by most selective schools in their selection process, to have the right academic level of students study together as cohort.