We measure system-wide power draw from the wall while running the 3DMark Time Spy stress test for 30 minutes.
We also use Nvidia PCAT to measure power draw of the graphics card only, with readings from both the PCIe slot and the PCIe power cables combined into a single figure. This provides us with significantly more accurate data to work with as it is measuring only the GPU power, and not total system power which is a fundamentally imprecise measurement.
Looking at total board power, MSI has boosted power draw pretty significantly with the Gaming X Trio. My testing shows an average consumption of 355W, almost 60W above the reference card and 40W higher than the Gigabyte Gaming OC.