There are a few threads with references to SDC including one on which I posted in response to a similar question - so apologies to anyone looking for St Dunstan's info for repetition!
My son is a year 7 academic scholarship holder, and is very happy. I think whether it's worth it depends how much financial pain and limiting of your life generally is involved, and what the alternatives are like from perspectives other than academic. Our son's peers at decent comprehensive are getting pretty rigorous coverage of the national curriculum, and I'm not sure that the purely academic benefits of SDC are necessarily worth it. But I think it's potentially worth it in other ways. For the kids, it's quite a small school with a warm and welcoming feeling, in which newcomers are treated in a very friendly way by older kids, especially those who share their extracurricular interests. The atmosphere is relaxed and teachers seem to know the kids well and are often involved in doing extra-curricular stuff as well as form stuff. As one of the more academic kids in his year (but there do seem to be a good number of bright sparks), our son seems to get a lot of positive attention, and encouragement. The Head, who seems fairly progressive and liberal in his values says he would hate there to be an archetypal Dunstonian - he wants them to be very much individuals rather than conforming to any public school pattern. Diversity is a strength of the school and it does feel quite connected to its inner city location, as far as that's possible for a private school in Catford.
They have a strong stated philosophy of trying to provide a rounded education, and I think they do a decent job. There is a 90 minute lunch break with strong encouragement to participate in the extracurricular Forder programme, including creative, sporting and community activities. In year 7 it would be hard not to do PE or sport 4 or 5 times a week, there are some excellent music groups with a very keen young music teaching team, and the arts are generally strong. After spring exams, all years go away for a residential adventure week, and then the year ends with an arts festival. Another strength (of private schools generally) is that they are not bound to follow the national curriculum (at least prior to GCSE years) resulting in I think a more interesting approach certainly to the humanities e.g. quite a lot of interesting philosophy teaching at the start of year 7.
So I think if you spend the money,, it would be for a broad education and a good chance your son would feel he's in a happy environment - probably more those things than a better set of GCSEs - and it must depend how hard it is to pay, what your other options are and how far he'd have to travel also.