Its all about your vram. WoW is a CPU intensive game and doesn't require much GPU power. It does however require a decent amount of vram on the GPU when boxing to store all the assets. For example, i run 20 off of a GTX 980 ti with ease and have enough vram to spare for a few more. That is however with all windows on low, and 1 on ultra at 1080p. The 1080 last i checked is going to have a mem clock and memory capacity (8gb) higher than the GTX 980 Ti and should have no problem with what you are doing at that capacity. Keep in mind i run my games with a resolution of 1080p and you run yours at a resolution higher than that and may not be able to do many more with one card. If you picked up a Titan, that would give you room to grow. Not sure how much more room though.

For comparison, my other system has a 780 Classified (Same thing) in it and could support up to 15 on low at 1080p. You say you have significant lag with just 5 and 1 on ultra.

Another thing to consider however is it may not even be your GPU. When you up your graphics from low to ultra the CPU is taking most of the hit. What CPU do you have? I build a lot of systems and do a lot of testing. I'll try to help the best i can with any questions you have on this stuff.