we have thrown 2 1080s into one machine, and have noticed the following:
rendering 1 frame on 1 gpu, it is 100% speed or say 100 frames of an animation in 100 minutes
rendering 1 frame on 2 gpus, it is 146% speed, 100 frames in 68.49 minutes
rendering 2 frames at the same time, allocated to 1 gpu each, it is 189% speed, so 100 frames in 52.91 minutes
The reason for it is if there are complex tiles on a frame, it is making one gpu render intensively for it, and one gpu just sits idle. waiting for the next frame.
the 1080ti is approximately 30% faster then the 1080. blenchmark is not a reliable source of benchmarks either.
We run the 1700x on our workstations here. It works fine. no problems. fast. reliable.