I 5-box on a 4K monitor with a 1070 8GB, a Ryzen 7 1700x, and 32GB of RAM. The 1070 is still not the bottleneck with WoW. I have the 5 instances spread evenly across my cores, with the 1 remainder logical core added to my master instance. I run my master at 7 in WoW, and my slaves at 3. 59FPS foreground setting, 21FPS background. My CPU is still the bottleneck because WoW is just so much more CPU intensive. My GPU breaks 50 or 60% usage in more recent and busy zones, but even running the higher resolution the 5 instances don't use more than 4GB of VRAM.

I guess the point is your 1070 is more than enough, especially below 4K resolution. If you want to upgrade, wait until the next gen of NVidia cards comes out, or the AMD Navi series since that's an actual brand new design made specifically for their new 7nm manufacturing process, whereas the Radeon VII is just a shrunk down version of their older design. I'm pretty hopeful the new Navi design will (eventually) be truly competitive with NVidia's high end.