There is an entire thread that discusses this very issue regarding GTX1080 SLI vs. Titan XP. The Titan XP won in the votes by over 80%. Reason being? Yup, the Titan XP provides more consistent performance regardless of the game being played. And max FPS does NOT tell the whole story. Even with lower FPS being shown onscreen in games, gameplay was still noticeably smoother with the single Titan XP in the vast majority of games. Also, native DX12 games (not those that were DX11, then patched to DX12) do not scale well at all when played with an SLI configuration. No such issues with a single Titan XP.
I owned a dual GTX980Ti SLI configuration and replaced them with a single Titan XP and can tell you first hand that what was pointed out in that thread is absolutely true. Yes, I could have easily bought two GTX1080s and ran them in SLI, but I knew all of the shortfalls by doing so. I have now owned the Titan XP for a few weeks and can happily say that I have found no need to buy a second one. I play games at Ultra settings at both UQHD and 4K UHD resolutions on monitors that have G-Sync (Acer X34 and Acer XB321HK) and gameplay is very smooth. Only Crysis 3 gives me issues, but that game's coding sucks (sloppy software devs) and was never properly optimized anyway. Turn off AA when gaming at 4K UHD resolution and performance jumps 30-50%! Anyone who does their homework knows that AA is an absolute waste at 4K UHD resolution anyways
Here's the thread for your viewing pleasure:
https://hardforum.com/threads/1080-sli-vs-titan-x-pascal.1905821/page-3Remember to keep an open mind as you don't own an Titan XP, and thus don't know first hand the difference.