Did the same here.
Just SLI an exact card if you really want Titan X Pascal performance.
A single 1080 is only 10-15% faster than a 980Ti. The 1080Ti is the true successor to the 980Ti.
The 1080 is just a more efficient card, which means better TDP/Thermals and has a few new features that will be used in games in a year or two.
Once the 1080Ti comes, 1080 prices will come down and 980Ti is pretty much end of life right now, but still a beastly card, it's just that it runs REALLY hot unless you water cool it (I'm in the process of doing that in the next few weeks since I'm doing SLI as well, but for now using a single 980Ti since my case is mATX and the fans run like a jet engine).
I feel sad, the 1080 FTW edition is beautiful and beastly card, but have to be pragmatic. Hopefully when Vega comes out, we can get it working on the Hackintosh side with Apple drivers and we won't be a lost cause.