Martin,
I have question concerning your heat dump test versus an in situ test, where one measures temperatures before and after the water block and the flow rate with an inline meter.

Are you worried about the accuracy of the flow meter? I see this and the performance decrease of an inline flow meter as the biggest drawbacks. However one would be able to log heat dump as a function of cpu load during their daily use. This might be useful data for people who do not run their cpu at full load. Also one could look at how heat dump varies with temperature, i.e. for the same operation what is the efficiency of the processor (how much current leakage) at different temperatures.

Also in your current setup what is the flow pattern? If you let the hot water enter the bottom of the thermos it would help to eliminate any temperature gradients. But from you picture it looks like the flow is pretty turbulent so the gradient should be small.