And it is a myth that a big waterflow is necessary, several tests have prooven that this is not the case. The water needs some time inside the radiator to cool effectivily down.
Correct conclusion, incorrect reasoning.
The idea that the fluid needs time to cool down is common, and is a result of thinking of the heat transfer fluid in individual parts rather than a closed system. Heat transfer is dependent on your delta T. Your delta T will decrease across the length of your radiator as a function of fluid velocity. If your fluid was moving infinitely fast, your radiator would be transferring the maximum amount of heat since it wouldn't have a temperature gradient. The same would hold true across your water block.
However, I will believe that people have tested much larger pumps then they need and found that they are running higher temperatures with the higher flow. Really high flow will have really high pressure loss through tubing, radiators, blocks etc... Your pump has a certain flow rate for a given head loss. The higher your head the lower your flow. Think of head loss as friction, which basically increases your fluid temperature. So the higher your flow the more energy your pump is heating the fluid with.
If this isn't making sense I'll try to put together a diagram or something tonight after I get off work, maybe with some of the math if I'm feeling ambitious and dig out my old textbooks.