Testing the block for thermal performance was fairly simple, once you realize that you have to measure VRM temperatures manually. As such, I installed an Omega NTC type thermistor on VRM 1 and connected it to an external display for a VRM temperature readout. TechPowerUp GPU-Z was used to monitor GPU core temperatures. The GPU was overclocked to 2 GHz, although with how GPU Boost 3.0 works it did vary by +/- 1 clock bin (13 MHz). Similarly, with core voltage being near impossible to set manually and fix at that point, it is best to compare the results below within the data set and not with other reviews elsewhere. For what it is worth, 1.20 V was set at Vcore using EVGA Precision-X.
Everything required was placed inside a hotbox, and the ambient temperature was set to 25 °C. Gelid GC-Extreme was used as the thermal paste of choice since not all blocks come with TIM included, and cure time was taken into consideration. Five separate mounts/runs were done for statistical accuracy and to remove the chance of any mounting-related anomalies. For each run, a 60 minute Unigine Heaven 4.0 run was done, and temperatures were monitored until a steady state was reached, after which they were recorded. A delta T of GPU core/VRM and loop temperatures was thus calculated for each run with an average delta T that was then obtained across all five runs. This way, the cooling solution is taken out of the picture. Lower is better here.
Note again, in case you skipped the previous page, that I have separate entries for blocks by themselves and with their respective backplates as sold separately. Those without a backplate entry specifically mentioned either do not have one available, have one included with the block itself or, in the case of the EVGA Hydrocopper, are installed with the stock backplate itself.
When there is just over 3 °C of difference between all the blocks in terms of GPU core cooling, you know things are hard to distinguish. The Watercool Heatkiller IV GTX 1080 is an excellent overall performer as it falls in the better half of the table here, with the backplate helping even further. The trend continues with VRM cooling where the Aqua active backplate combination is the only entry that is significantly better performing than the rest and the Watercool combination still being in the top three. While VRM cooling below a certain number does not do much for the GTX 1080, look at these results more of an indicator of where things would be for other cards where VRM cooling can help more.