Now, how do these duct mods inmprove cooling? They're just a tube...
Quite simply really....what do you think would push more air...one fan...or two? Old saying goes, two heads are better than one...yeah its a tube and its gonna push in air at an already accelerated speed then hit ANOTHER fan which of course would propell it even more....but you gotta have a strong 60mm motor cause the force from the 80 could burn it out....if you had an 80mm counter-clockwise spinning fan and a 60mm clockwise spinning fan that both blow air the same direction...you'd have a compressor basically and it's really improve that duct mod......with blades spinning opposite of each other, but still pushing air the same way it creates a compressor action between the fans and then the air coming out the bottom is even more 'furious'.
They're just a tube....come on...just think about it for 5 seconds and its pretty obvious that it'd be more efficient than just the single fan.
The one on CPU fans is so to distance the fan away from the heatsink a little so the air can fill up the entire tube and do a more direct flow of air because there is that dead spot where the motor is...this essentially takes care of that dead spot.