Having more than one GPU in your system will most certainly decrease your render times if you are using Vray RT, which is a GPU based render engine and uses no CPU power whatsoever.
However, due the new architecture of the 1080, and its faster clock speeds, one 1080 will still render much quicker than two 980's. What you must also remember, is that the CUDA sdk hasnt quite been realeased yet for the 1080, so most of your GPY render engines do not support the cards yet. This will hopefully change soon.
I am in the process of creating a render server with two 1080's, and no need to SLI bridge them either, the engine will pick up both cards and use them to render over the PCI bridges. Nice thing with the 1080's as well is that they have the NV Link technology, so both cards can share each others memory and you basically get 16 GB of VRAM instead of just the 8 GBs.
And yes, the 980ti has more CUDA cores, but the new architecture of the 1080's can do more with less cores. You could however pick up the 980ti's cheaper now, and still get good renders out of them.
Adios.