They both have the same amount of vram, 8gb.
I don't know what kind of performance you're expecting, but I can share my experience.
I have 1080 in SLI and boxing 5 characters on 4k resolution.
It's really not how this works, but if everything scaled the same:
5 chars at 4k is as many pixels as 20 chars at 1080.
I have 2 cards and if the scaling was 100% (ie, 2 cards give exactly double the performance) then 1 card would give the same performance I get for 10 chars at 1080.
I play max settings on my main, and lowest settings on all others and get 60fps on main.
I am going to say this again, this is NOT how it's going to behave in reality, but it might give you a very rough idea.

My cpu is a lot stronger (5960x @ 4.5ghz) and wow is running from the 840 evo ssd with 32gb of ddr4 ram.