JWFokker, sorry, I did not realize you had to sign up even to read that board. Cathar has done an in-depth multi-page reference post titled -
How much pump is too much? How much is enough? [Edit: Note there is now a
duplicate of this thread posted in the ProCooling forum].
I think it would be disrespectful to quote much of it here, especially with the risks of quoting out of context to the reference charts and data. However here is one small snip.
DO NOT select a pump that draws more than 50W of power. Performance will not improve further and the pump will be drawing more power than is needed, effectively being a waste of electricity which just shows up on your power bill. Pumps drawing much more power than 50W will actually make your CPU hotter.