I use Sidi Dominator 7 SR mtb shoes (a narrow shoe, so a bad fit for you) with SPD cleats and Shimano A600 road pedals. No worries about power transfer; you'd be hard pressed to tell any difference from a high-end carbon sole road shoe and pedal setup. But there are a lot of mtb shoes that are little better than tennis shoes, so make sure you get a pair with a stiff sole to provide proper support. The Dominator 7's are very walkable.
I think for foot width the question is exactly how wide...