The fans volume have a fairly significant impact on thermal performance, it's something in the neighborhood of 50% less fan volume is 40-45% less performance. Here is swiftech MCR220 for example:
The top lines are with 162cfm total (81 each) air flow, middle is 95 (47cfm ea), and the lowest is 67cfm (34cfm ea).
So at an assumed 1.5 gal/min, you could dissipate 300 watts at 162 cfm, and 150 watts at 67cfm.
So it's not a linear relationship there is a curve to it, but you loose about 50% of the performance when you cut the airflow by 60%.
Here is thermochills graphs,
note they are using different fans so you can't compare apples to apples, but in regards to fan speed you'll see a similar relationships...the PF is 100cfm panaflows, 12m are 73cfm deltas. The important point to note is that at 1.5 gpm at 12v panaflows 400 watts dissipated, and at 7v, you get 270 watts: So cutting the voltage down by 40%, you get about 35% loss in performance so it's very similar.
Bottom line is running low cfm fans means you better add some radiator capacity because you may only be getting half the performance out of it. Or if noise is not a concern, using some 100cfm fans will provide you quite a bit more performance with less radiator if you need it. My preference is less fan and more radiator for less noise, but it's a balance on what heat you're dissipating, the radiator space you have, your budget, and your noise preferences.
Also as already noted, some radiators are better for high cfm fans, and some are better at low cfm. The thermochills are king in the low cfm market (you pay for it thought), or something more budget oriented like the swiftech mcr series, HW BIP or GTS are also good low speed rads. Something like the HW GTX series need some strong high speed high pressure fans to get the most out of it. I've heard heat cores also need some good fans to push through them well.