DX12 for WoW is just DX11 in a wrapper.
I just upgraded a system for a fried this weekend gone, from a 1060 to a 1070TI clocked to GTX 1080 speed, and you know what the bottleneck is? It's the pants old engine that is un-optimised for modern hardware. It runs like dog doodoo on Ultra (10 in the settings) and spending ages tinkering didn't help that much since the GPU was sitting at 60-70%, and the CPU around the same. Runs 100+ FPS in raids, and BG's, but certain parts like Dala, and flying over Suramar just made it nose dive to the 40's.
BFA is out 14th August, so I can understand why you might be eager to upgrade, but with WoW the best thing going it Adaptive Sync not pure FPS, since the game can't do it. Do you have a G-sync monitor?
WoW is CPU limited in cities. Crank up the Ghz, it makes a lot of difference.