As mentioned above - unless you already have the *best* GPU available, and thus no choice, SLI is often a poor upgrade (for games). The vendors are dropping support for implicit multiadaptor in anything but benchmarks (as the complexity of splitting work efficiently in the driver is huge, some engine architectures make it near impossible to scale well), and game devs aren't willing to put in the work to do the same on their side with explicit multiadaptor (again, outside of benchmarks) due to the extra development costs and vanishingly small target market.
You'll probably do much better on average selling one of the 1080s and getting a 1080ti or titan (or whatever the next generation top gpu is going to be)
You'll probably do much better on average selling one of the 1080s and getting a 1080ti or titan (or whatever the next generation top gpu is going to be)