Don't listen to the "it's overkill" nonsense. I have 2 GTX Titans (the originals), which I overclock +200-300MHz, and some games still give me trouble at 2560x1440. Although, I should note I crank up AA a bit, which I find essential. GTA V drops down to the 40s when I visit the countryside. Since 2 GTX Titans are more powerful than a single GTX Titan X, I conclude that a single GTX Titan X is not overkill for 2560x1440. I would actually recommend 2 Titan X's if you're aiming to use a lot of AA and you want the highest possible FPS for shooters (assuming you have a 144Hz 2560x1440 monitor).
Why do so many people on here write off the highest-end cards as overkill? Is it because they don't have it? I guess it's a way to justify...