Interesting discussions with Asetek. I think that the answer is more complicated than saying that higher velocity is better or worse.
You can speed the water up with a single pass thin-tubing radiator and have worse performance than a heatercore. On the other hand, you can have a large tubing multipass radiator that performs worse than a thin-tubed multipass radiator. You have examples where velocity is not the primary problem.
Instead of focusing on "slowing the water down" in the radiator, focus on keeping the water close to a heat exchanging surface for a longer proportion of it's time traversing the loop. Both elements have to be there: 1) high proportion of time and 2) quality of heat exchange.
The slow vs. fast radiator flow debate (to me) seems better answered by looking at the costs of impedance to water and the amount of surface area that the water (not just the air) is exposed to.
Blah. Feels like I'm talking in circles on this one.
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