Sadly scaling gets worse, the more cards you add. The second card is like 90-95% more performance in optimized games. The third one only adds like 40-50% of what the second card gave you and a fourth one sometimes even gets you less performance.
SLI 980tis should be able to run stuff at that resolution at 60fps most of the time, but there is sadly no way to get to stable 144fps on these kind of resolutions at the moment.
Maybe the next generation of GPUs will take care of that.