Fans are commonly rated at two extremes - the volume of air they can push when there's zero restriction (aka maximum airflow), and the amount of pressure they create when they're completely blocked (i.e. static pressure). Real-world situations fall in-between those two extremes.
For a case fan, there's usually not much restriction involved, so a more useful number is the former - the maximum airflow. But for a heatsink or a radiator, the dense cooling fins represent a fairly significant restriction, so you should be more interested in a fan that generates a lot of static pressure, in order to keep the moving, even through the restriction. In the above example, Noctua's S-series of fans are specifically designed for case usage, to the point where they strongly sacrifice performance on radiator and heatsink (and Noctua actually warns about this). Noctua's P-series are designed the opposite - maximum static pressure, for best performance on restrictive applications. Of course, whether they manage this is up for debate, and if they do, whether the improvement is worth the price premuim, is a whole differente debate.
Frankly, when it comes to absolute performance, fan design is mostly a secondary issue. It's pretty darn hard to make up for the performance that raw rpm's provides. It's when you're not willing to put up with the accompanying noise that you start looking for the edge elsewhere.
