That's right, the above formula is just the power of the wind, the amount of energy you are able to harvest from it does depend on the characteristics on the blade. The swept area is still a key factor. (see Betz limit for another interesting fact)
You are right about the second part, those 'tunnels' do catch all the wind of their area.
Ok, I think you got my point. To sustain it: "Theoretically, an infinite number of blades of zero width is the most efficient, operating at a high value of the tip speed ratio. But other considerations lead to a compromise of only a few blades.[22]" http://en.wikipedia.org/wiki/Wind_turbine_design#Blade_count
You are right about the second part, those 'tunnels' do catch all the wind of their area.