Actually, if you want to introduce the airflow as a factor, you really can't compare it to a waterblock anymore.
The fluid velocity is going to depend on the size of the opening. In a rad (heatercore), multiple passes are common, making them less restrictive. A waterblock is most often the biggest restriction (Airspirit is now the exception!).
You're right though, a rad will perform very nicely at a low flow rate, lower than a block. Why would you want to reduce the efficiency of a rad though?
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