I have my 3 rog swifts running off one 980ti classified right now and the performance is not horrendous, quite surprisingly. I can actually play most of my games like BF4 at around 40-50 fps with AA down to 2x. (I should probably just turn it off.)
 
Again this is at 7680x1440 which is a huge resolution and the VRAM in games varies greatly. For games like Dying Light even on one ROG swift it consumes mostly all of my 6gb of memory but it runs at great frames. However when I play on surround my frames tank, if I got a second 980ti the sli performance would surpass the Titan X because of the clock speeds of the gpu are higher. However if you played a game like Dying Light, GTAV, or Witcher 3 with ultra textures on you are likely to hit the 6 GB cap, this will slow down the 2 gpus working together because they can only work with what is loaded in video memory. In this event you would see better performance with 2 Titan Xs but not by a whole lot. 
 
7680x1440p is a cutting edge resolution. You can game on it currently but not with everything turned to max especially AA and Ultra Textures. I am still debating either to get another 980ti OR wait for another high level card from Nvidia to come out with more VRAM which WILL happen. One TitanX would not do the job for this resolution, a 980ti is going to perform better. 
 
I will try and post more when I find out but without a 2nd 980ti at my disposal I can only speculate right now.