It's not just airflow. It's temperature. The blow in arrangement means the air being supplied to the case to cool everything BUT the CPU has already been heated up by the CPU radiator. How much?

My Q9550 has a max TDP of 95 watts, according to Intel. The fan on the Corsair moves a max of 59 CFM. That's about a 5 degree F rise in the air temperature. (CFM * 1.08 * delta T(F) = BTU/h, BTUH/3.414=watts)

Probably not a big deal but if you're overclocking and changing the airflow in and out of the case, its worth knowing. The temperature rise would change in direct proportion to the wattage or the airflow.