Sli depends on the game engine and optimization of code. Some games are so badly optimized for sli you actually get negative performance, better performance with a single card sli disabled, and some games you get 70% or better performance.
Not long ago sli 970's would run @$600. A 980 ran @$600. The 980 averages @20% better than a 970. The newer games have better sli support so sli 970's averaged @50% gain. So sli for a 970 was a much better option than a single 980 for the same rough price. The 780/780ti/970 are pretty close in performance, at best sli 760's will hang, pretty close, but generally sli 760's will be somewhere between a 770 and a 780 in performance. With upgrades to single gpu power like in the 970 or rx480, sli 760's 3Gb won't do much better than 1080p, and that's with a good game.