And the comparison with GS?
It is likely that the avergage Grammar School outperperforms the average Indie. In fact it is almost a mathematical certainty.
There are about 150 GSs, and they are all academically selective. Every kid in there is at least in the top 20% in their peer group. In some schools (such at Judd and Tiffin) virtually all will be in the top few percent of their age group.
There are about a thousand private senior schools (or is that all including preps?).
The important point is that only the best of them are truly academically selective in the rigourous way in which GS kids are selected. The main criterion for enty is money, not ability. Many of them take all-comers, and some others specialise in special needs.
As a catchment group they will vary from very smart down to the rich and thick. How could they possibly outperform the GS group?
The 10,000 or so state schools (which include GSs) vastly underperform the Indie schools at the top end (3As), and the point of the article posted above is that for all the trumpeting of Education, Education, Education, that gap has widened dramatically.
IMHO it is the publication of league tables that has had this effect. Schools that rested on their laurels (eg Harrow, Wellington) were shown some years back to be quite mediocre, and have had to up their game.
With no real sanctions against mediocrity, and fewer resources, the State system has not responded in the same way.