We subjected every card to a gauntlet of synthetic benchmarks and real-world games to try and answer the question, measuring power use all the while.
Middle-earth: Shadow of Mordor isn't too punishing on graphics cards... until you load up the optional HD textures add-on that isn't even recommended unless your GPU has 6GB of onboard RAM. Then it hits hard, which makes it great for testing high-end graphics cards at high resolutions.
The game was tested by using the Medium and High quality presets, then by using the ultra HD texture pack and manually cranking every graphics option to its highest setting (which Shadow of Mordor's Ultra setting doesn't actually do). Attempting to change resolution or graphics levels while using the Radeon R9 295x2 hard crashed our system every time, so it's not represented here.
Sniper Elite III is no graphics powerhouse, but it's an AMD-centric title that serves as a counterbalance to Shadow of Mordor's Nvidia leanings.
Alien Isolation scales well across hardware of all price points. We recently used it to test the mettle of high-end graphics like the Radeon R9 295x2 and Nvidia's Titan X at high resolutions.
We also tested the cards with a pair of traditional benchmarks: Unigine Valley and 3DMark Fire Strike.
Power efficiency and temperatures of these GPUs.
Note that this is the power use of the entire system under load, not just the GPU, measured at the wall using a Watts Up meter during a Furmark run. Baseline system power use varies between 73W and 80W depending on the graphics card. The temperature measurement is of the graphics card only, however.