Well quite obviously it makes the ventilation of the case a little more effective since the CPU fan is sucking fresh air, but how much is just imagination and how much is real is another thing...
Normally, on case like this the only thing that gets new air into the case are the rear fan and PSU fan creating a lower pressure inside the case, which drags air in from every hole and corner of the case, but directly pushing air into the case is a different matter altogether. I never did any benchmarking (and damn me if I can be bothered into changing the fans and removing the air duct to make experiments on CPU temps), but if I recall correctly, the CPU idle temperature was lowered somewhat, but compared to the retail fan, the idle temperatures lowered about 5-10 degrees Celcius, but I can't say how much of that is caused by air duct, how much by new fan increasing the air flow through the heatsink blades, and how much is from cleaning the heatsink from accumulated dust. Ambient temperatures dropped a few degrees too if I recall correctly, and that should be a direct result of the duct making the CPU fan an intake fan as well.
Right now, SpeedFan is reporting CPU temperature to be exactly the same as ambient temperature of the case air (35 deg Celcius), but I can't know how accurate the temperature sensors are and where they are. One of my MOBO sensors currently reports (or is interpreted wrong by the SpeedFan) 116 degrees Celcius and I suspect my PC would have become a toast (or toaster) long ago if it were accurate...
At any rate it was fun to make and at least I can imagine it working.
