maybe dx9-dx10/11 performance difference is engine dependent too, in another engine I realized significant fps boost (jumped from 22-24 to 38-41 in average) when the same c++ code was compiled in dx10/11 mode (of course using the same level and settings). I have not tried out unity4 yet.
I'm not sure if it's engine-dependent but I've already said what I know:
my brother's application (programmed in C#) runs 5x faster (900fps) with DX11 and needs just 1/3rd of the ram usage (mostly geometry-data) comared to dx9.
