No one is buying the TX for lowly 1440p, it's 4K all the way. OC TX is a beast but still does not justify the daft price tag. If this had come in at £600 Nvidia would probably rake in more profit than charging the $999 through pure sales volume.
I didn't notice any dodgy frame drops in DF vids.
Personally id say 2 are needed for 4k, even with my 295x2 being faster than the titan x id still probably want another gpu in there for at least 3 way crossfire.