With the new Legion graphics, I decided to upgrade because I run 4K (couldn't max many games anymore), and I was no longer getting a solid 60+ FPS on the 10 preset with no AA enabled.
My previous build was a 3770k @ 4.5GHz with 32GB of DDR3 1866. I upgraded that to a 6700k @ 4.6GHz and 32GB DDR4 3200. There was no difference in the game, I tested multiple zones and did the same flight path loops. Safe to say, if you have a decently overclocked processor it's probably not the bottleneck anymore, even if it's a few years old.
I then upgraded my Titan X (Maxwell edition) to the new Titan X (Pascal edition) that just came out yesterday. I overclocked it to +200/+500 just for giggles. The difference is huge. I went from seeing 45-50FPS in Hyjal due to that very thick grass, to around 98-114FPS. Every zone runs like butter again, even on the new 10 preset. I enabled 8x MSAA (overkill for 4K, but why not if you can?), and I am still maintaining 60+ FPS in every zone while flying around.
So can we put the "WoW is a CPU bottleneck" claims to rest? This is no longer the case, and honestly hasn't been the case since WoD launched. Your CPU will only be the bottleneck if you are running an old processor at stock clocks or a super low resolution.
Hope this helps someone that is planning to upgrade for Legion.