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But as far as I know 1500mhz is a very rare thing, normally they are about 1400-1450. 1500mhz titan X would be a lucky chip + watercooling.
This is part of the problem with all these comparisons. Everyone is often comparing the reference 1080 to the max overclocks of the 980 Ti and Titan, as if all Titans and 980Ti's will give you 1500mhz or more out of the box 100% stable in all games.