You got nothing to worry about for GTX 260 sli as long as the game is supporting sli........I mean the scaling is not perfect, but good enough to match with GTX 470 performance in most of game...
Dx11 is just a joke so far, few games are supporting it and no much real difference even in most of time, unless we are talking about apparent drop of FPS
By the time DX11 shines, I believe GTX 470 cant even run game well already.........