I think you way overestimated K80 performance when my 4GB GTX 960 back in the day could trade blows with a K40, which was a bit more than half a K80. In a straight memory bandwidth fight, like Transformer model training, the 1080Ti is going to win hands down even if you have perfect scaling acorss both GPUs on the K80 and that's assuming it doesn't get hamstrung by the ancient Kepler architecture in any way at all.