I tried that, but sometimes you run the risk of not being able to find the same model/make of display card for esthetics...
From the few examples of SLI benches from 1080's I've seen... it seems on good SLI profiles the gain is 90% average, much better than previous generations.
@RadicalAns, I think SLI 1070's might be the way to go if there's a budget involved, but SLI 1080's will definitely scale better and give more future proofing with DX12 mGPU support.