Ideal order has the CPU coming right after the radiator. However, the head loss from bad tubing runs makes it more important to have a good layout for the tubing. I believe that you lose more efficiency from a kinked or sharply bent hose than you do from putting the CPU before the radiator.
Saying all that, I think you can have your cake and eat it too. Just put the rad before the CPU and make a neat and tidy tubing run.
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