Air flow requirements can vary so widely. Chris' question about dust collection for carving on the other thread (very small shop) kind of underlines it.
You can get quite decent dust collection with low air flows on a tightly hooded machine - where a well designed hood can be positioned to intercept flying dust, and where the fairly high air speed require to collect it only has to be maintained over a very small open area. The Festool example mentioned above.
The problem is that this is not possible on so many machines. A wide jointer or planer, or a decent sized table saw with a leaky bottom chute and wide top guard making angled cuts is another. Even quite small band saws can be another - if as is often the case you can't place the pickups so that they create the above high speed air flow through a small area, then the CFM/air flow requirement needed to do a decent job can be quite large.
Worst of all is perhaps the downdraft sanding or carving table, or fume hood type set up. To get these working properly you don't need much airspeed (maybe 0.8-1.5ft/min - don't know what the official recommendation is), but you need to maintain it over the entire open area of the table or the hood. Which for even a smallish 3ftx4ft = 12ft2 runs up to 12x0.8x60 = 576cfm. 1,080cfm for the higher end of the airspeed range.
What I'm driving at is that the above range of airflow/CFM numbers takes you (in a small shop scenario) from maybe vacuum levels of flow (on small well hooded hand power tools), through the 450CFM or so that's the minimum for low pressure systems on most floor machines, to the 1000CFM plus of a large Pentz or Oneida type dust system.
Pressure capability is the other variable - tightly shrouded and small ducted hand tools and the like need much higher (vacuum like - say 100in WG) pressures than smaller floor machines at say 12in WG/450CFM if well hooded. Larger floor machines/less effective hooding may require 15in WG and 1,000CFM plus. Against that a downdraft table or cabinet can be power by a low pressure axial flow fan capable of only maybe 0.5in WG but shifting lots of CFM - if that is you don't mind exhausting outside to avoid the need to filter and/or cyclone, and pulling in fresh air to compensate. Recirculation probably means with the extra pressure drops etc going back to a large Pentz/Oneida type system at the above 15in WG/1000CFM plus.
There's no easy answer to all of this, but it very much underlines the need to be clear about the nature of the machine/dust collection application before (as we often do) rushing to the 'I've got an XYZ' discussion. When requirements get lost then all the human bias and selective perception kicks in. 'You can't possibly need that much CFM' etc ..
ian