It can handle gtx 970 SLI with ease though, what makes you think it will be bottlenecked?
GTX 1080 is about 30 % better than 980ti and about 25% than sli 970s. Putting them in sli would be massive better than sli 970 by about 85%. SLI 970 and SLI 1080s don't belong in the same conversation.
Bottleneck will be circumstantial, you won't be held back at all by CPU power in all the games you play. I would only suggest SLI GTX 1080 with even more powerful CPU's though I have to say, i7 Haswell or Skylake.