iphone built quality setting

Hi, Does anyone know is there any difference in the final build for Unity Iphone by setting the different quality settings( Fast, Fastest, Fantastic etc..) . One of my scene got more than 20K triangles and when I switch the setting to Fast one, the polygon count drops down to around 10k.. But the framrate on the actual iphone doesn't seems to be good enough. How can I improve the actual framerate? I've tried occulusion culling methods and it seems not to be working on somee export models from Fbx format.

Quality settings do not affect iPhone builds.