Well, if there is a leaf blower that can blow 640 MPH for real, I would not buy it. Why? Because the air speed is so fast that the leaves would quickly slip out of "blowing zone" and fly in the air. A slight breeze will bring the leaves away from you to a different spot, that you might have already cleaned up. So this would be inefficent machine.
CFM is a short word for cubic feet per minute. This is the amount of air that it blows. The more CFM, the more leaves it can blow. About the MPH factor, in theory, the more MPH the blower can push the air, the farther the leaves will go. But that is not always necessary true. It would not be efficent if the leaf blower has the higher MPH ratio than the CFM ratio. The blower can blow the leaves farther, but you need more air (CFM) to blow it without having leaves fly out of control.
If I were to go out and buy a leaf blower, the first thing I would check on is the CFM, then the MPH that it can display. In this area, I have alot of big maple trees that drops tons of leaves. So I would be more concerned about blowing more leaves, than the distance I blow the leaves to. Yes, in some case, I would like my leaf blower to blow the leaves farther but that would be hard to make a big pile of leaves in a small radius circle.
Hope that you get the idea. It is only my opinion.