Stu, I would gear it down or knock some timing off. A fan stuck on the back of the car will upset the handling. I know it is not that heavy, but it will make a difference.
For it to have any effect it has to be close to the motor which means having it on top. Any position more than 5mm away from the can will have minimal effect on temps. I doubt it would reduce temps by more than ten degrees over the run.
If this was such a good idea, everyone would be doing it by now. No one does, so that tells us something, eh?!!
