He throttled the 1080 to get clocks comparable to the 980ti, then made some calculations to adjust for shader count and he posted the results in the video. The 980ti was actually slightly quicker when the 1080 lost its clock speed advantage.
Of course its not quite as simple as this as nvidia managed to boost these clocks whilst reducing voltage which in turn reduced heat and power usage. But the clock for clock performance is more or less equal.
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