You should test it then and report your findings. I did something similar maybe ~10 years ago with a duct going to a heatsink; it worked great. I would set it up with the rad as the intake unless you have a GPU that exhausts out the back. The effect of convection inside the case is small enough to ingore. Just make sure you have decent airflow around the outside of your case so that the warm air doesn't get sucked right back in from the top.
Edited by CastleBravo - 1/29/13 at 7:32am