And that's really the key point, choose a fan that moves enough CFM. Not dogging on the black magic fans themselves, as long as you get one that moves enough CFM they should work great.
The contour fans supposedly move about 3k cfm on low speed and 5k cfm on high speed. Fan flow ratings can depend a lot on the flow test itself, so I wouldn't say that's an exact number. On my car I know that I've only had the high speed come on a couple of times, and both were situations where I was out stuck in traffic in 110° weather. The rest of the time the low speed is more than adequate, and even in those cases the high speed didn't run for more than a few minutes at a time.
So what I usually say is that for most folks and how they use their cars a 2,500 cfm output is probably enough, unless you're in a hot climate and intend to drive your car in pretty extreme situations. Obviously that also depends on the radiator being used, how wild the engine is (more horsepower = more heat generated), etc, etc. But I think most folks would be parked long before they were stuck in traffic in 110° weather.