http://www.babeltechreviews.com/battle-titans-pascal-titan-x-vs-maxwell-titan-x/3/
(I did it this way because I couldn't find a great comparison of Titan X(M) to GTX1080, but this will be close enough)
A GTX1080 would sit in between these two Maxwell cards. At 2560x1440 it's probably about a 30% boost max from a GTX1080 to a new Titan X(P).
So it only looks like you'd get 25 to 30% boost on average going to a GTX1080.
For example, look at Alien Isolation. If we assume:
Titan X(M) to GTX1080 = 30% boost, then
GTX1080 to Titan X(P) = 30% boost then
1.3x1.3 = 1.69
It works out close (at 2560x1440 the boost from Titan X(M) to X(P) was 67% so it means about a 30% boost is what you can expect going to a GTX1080.
*also look at the FPS value. If you can get above 60FPS anyway is it worth it to you?
I jumped up from a GTX680 to GTX1080 and performance in some games was tested at 3x (i.e. 20FPS to 60FPS), but with careful TWEAKING of game settings I could play almost every game with excellent quality at mostly 60FPS.. there is DIMINISHING RETURNS for quality at the higher settings...
(in fact, some games drop to HALF the FPS at Ultra vs HIGH yet you may have a very hard time telling the visual difference)
So basically I recommend turning down a few settings, then wait for a GPU that gives you a much better jump in performance. If it was me I'd wait until 2019.
I'd also look into getting a GSYNC monitor if you don't have one, or if the monitor is good now (even if not GSYNC) maybe wait for GSYNC + HDR and prices to drop closer to $600USD for a good 2560x1440 screen (or 3440x1440).