I was chatting to a friend today about a girl who got 10 A*s at the local comp and her friend who got 5 As, 4Bs, 1C at the nearby selective private school. It has always been possible to get great results from a reasonable or good comp. But it's not all about the exam results. For me it's about the climate in which they learn and how those results are achieved. If you think that overall, results aside, the private school will suit her better mentally, emotionally, academically and prepare her better in every way for the future, for her career and her life, send her there.
With the obvious caveat that 5 As, 4 Bs and 1C will exclude that child from many academic and career options, so some parents may feel the fees, particularly at a selective school, were a little high for such an outcome. Depending, of course, on the aptitude, personality and attitude of the child in question, which can never be factored into these arguments successfully. Sadly we can't run controlled experiments with our children and life is always full of 'what ifs?'