"Though we mostly focused on games that support Nvidias multi-GPU technology, these also happen to be some of the most popular PC games released as of late. On average SLI enabled 63% more performance or 71% if we ignore Ashes of the Singularity. We'd call that a solid score for SLI scaling.
Granted, this may not convince many to spend another $600+ on a second GPU but in a good number of scenarios at 4K, it pushed performance from playable to smooth-as-silk."
Steven Walton - TechSpot.
Nvidia GTX 1080 SLI investigation by TechSpot - Introduction & Conclusion.