Yeah I know it won't double FPS but could sure result in some solid FPS gains as compared to GTX 780.
If his other parts wont prove to be bottleneck, he can get around 40% - 50% more performance, not 70% increase, but still much more than a single GPU.
Actually when playing a game at those settings and resolutions it can make a bit of difference in FPS. As said, GTX 780 is just a overclocked GTX 680 (With minor tweaks) and GTX 680 gets about 30 FPS on Crysis 3 on ultra at full HD. So playing the same on a 1440p display would get it somewhere in between of 20 - 25.
At that time, GTX 770 SLI can make a difference. At the moment I would recommend two configurations -
1) GTX 770 Single 4 GB.
2) GTX 770 SLI 8 GB
Buying GTX 780 over 770 is a big mistake because it is just a bit more powerful but is much pricier.