Watch Dogs 2 too. There have been cases where people with GTX 1080 struggled to play the game as it kept stuttering. The issue turned out to be that their GPU was barely used, as their PC was bottlenecked before it got to the graphic card. They were using something like 1600hz ram speed. WD2 has very high CPU usage, but turns out ram matters a lot too.Well this days at least from my tests i do see 2133 Mhz as a minimum for RAM, if your after good system response times. Gaming wise there's very few games where RAM speed makes a difference, Fallout 4 and Overwatch are the only ones that come to mind, though i think there where 2-3 others as well.
I'm using 2400mhz ram and didn't encounter this issue, my game running faster than theirs. But 2400mhz ram with the GTX1080 would give even better results.