Yes I have a similar question and this thread didn't get very far into answering the OP's question. Yes- Legion is far more demanding than the last expansion was (as well as the graphics engine doesn't work well with the API or something cuz everyone is having these issues).
+25% isn't that much.. considering a 1080 gives a similar jump from a 1070- but I'm sure it scales better at 4K than a single 1080 does. I have heard a few claims that SLI can run +50-60% performance- of course those are at different resolutions.
I have a 1070, 6600k, SSD, 16gb ram and 2560x1080 Ultrawide Monitor. Maxed settings, including SSAA x4 + CMAA, I am getting 35-50 FPS on average. I am wondering if it's more likely I'd see a higher average FPS with another 1070 or a 1080?