I think you've hit it actually areyoubeingserviced.
There is a massive difference between being well qualified and well educated. My dd is probably top 25% rather than top 5% (remember we are in the London microcosm). The comps would have got her to a certain qualified state with probably 8 or 9 B+ GCSE. Unfortunately she wouldn't have come out of it particularly well educated. She probably wouldn't be able to construct a grammatically correct sentence (and I appreciate I have opened myself up for mass criticism here) and she probably wouldn't be able to convert a fraction to a decimal with ease. At the end of the day, she can do those things without being an A* student.
My DH has the brain the size of a planet and went to the local comp; he felt for years and years that he had to compensate because somehow, in spite of having a mother who had been a primary deputy head, he had to work twice as hard to make sure the absolute basics were right.
I work in HE and never fail to be shocked at the poor standards of basic skills many of our students and employees appear to have absorbed. And some of those employees are post doctorate. Those problems don't seem to be present in those who have received an independent education and I think that is a dreadful indictment of the status of state education in the UK at the present time.
I have one academically gifted child and one child who is probably top average. We have paid and the benefit, I believe, is that my children will come out of the other end well educated as well as well qualified. It is very difficult to quantify but I really do believe the benefits will be reaped as they get bigger.
I am old and wasn't regarded as university material even at grammar school but I truly and honestly believe that I was successful because I was well educated and was able to write relatively well and deal with numbers relatively well. My DH has the brain the size of a planet and is exceptionally successful but at 29 (many years ago) needed his confidence bolstered, his grammar corrected and me to teach him some of the basics like shaking hands and making small talk. He went to Oxford from his comp btw and took a first. He has been passionate about the fact that he didn't want his children to feel slightly inadequate when they got there. One is going; one will not but the one who will not is nevertheless very well educated. That is what I think we have paid for and I think it is unspeakably sad that a good all round education doesn't seem to be available to all anymore, regardless of ability. Although that is, of course, a much wider debate than required by this thread.