Their data suggests that the air gap provides minimal resistance to the flow of heat. As for the clearance, they've stated that unusually high-precision manufacturing is not required for the two surfaces.
Re: clearance, while .001" clearance isn't something I'd want in a design particularly, modern machine tools are good enough that you get flatness and surface finish good enough to not have problems at .001" clearance basically free on any machined surface.
Their data suggests that the air gap provides minimal resistance to the flow of heat. As for the clearance, they've stated that unusually high-precision manufacturing is not required for the two surfaces.