It becomes a matter of definitions above 1080p and that applies to specific games even more so. I've seen benchmarks where at 4k, with the same build, the lowly 270x got better fps than a titan-x, but that's just the differential in game engines, optimization, api's etc specific to that game, in every other game tested the titan-x blew the doors off the 270x at 4k. In dirt3, titan-x sli actually got worse performance than a single titan-x, but again, that's specific to that game and a few others.
Generally, for good playable fps at ultra settings, for most games, 1080p - 970/390, 1440p - 980/390x, 2k - 980ti/fury x, 4k - titan x/295x2 are minimums, sli/xfire changes that somewhat, but only in games that have optimized support for such. In BioShock Infinite @4k, sli is so dismally optimized that even sli 980's is only seeing 20-30 fps whereas CS:GO is off the charts with SLI 970's at any resolution.