Actually, the catchment for Dunraven is widest for the top bands and smallest for the lowest, because of the number of high priority applications that are made from band 3 children. And I guess the middle two bands tend to be 'average' so many children fall into them.
Certainly my experience of where children come from bears this out - wide catchment for top band.
Anyway. My dc are at Dunraven and I have relatives and many friends in Elmgreen, including kids who would be in the 1:1 Dunraven band, and they are doing very well in Elmgreen and are very happy.
Behaviour is better in Dunraven ( I have heard this form kids and from teachers who have taught on both) but that might change as the new head at Elmgreen gets a hold. They have different feels and ethoses. Both good schools.
Also, Elmgreen uses setting and Dunraven uses streaming (although they do set as well, they move kids about for English and Maths according to specific ability) but if your DS is strong in some subjects and less so in others setting will be better.
The 2 schools have a joint folk band.
For a high ability child, personally I would err on the side of Dunraven, given a choice, but Elmgreen is a good school. You could stay on the waiting list, but if Dunraven didn't exist you would have a v good option in Elmgreen, iyswim.