Although not the same as setup as yourself, its pretty similar, with the same motherboard. I've recently moved from a 7970 CF setup, so a touch worse than you 780ti, depending on your GPU OC.
Can't comment on your relative gain without having had the same card, but I can tell you that since getting the card a month or so ago, I have been loading up everything I have and putting the GPU through its paces. I game at 2560x1440 60hz and with a lesser i7 930 at 4.2
Since getting the card I have played the following with a locked 60fps, bar the odd hitch here or there:
Batman Arkham Knight
Gears of War 4
Witcher 3
CoD Infinite Warfare
This is with details at max, with the only exception being maybe turning down and particle affects (if necessary) as they tend to be CPU bound.
Long story short, using the 1080 on that platform doesn't seem to completed obliterate its performance. In fact, on the well coded ones above, such as GoW4 and IW, I am able to oversample to nearer 4k and still maintain the framerate I want. Basically its performed admirably.
I'm happy as I think this card will last quite while. 2560x1440 seems to be a sweet spot, as when engines become more demanding then the native resolution is detailed enough to look good but not too hard to drive, but oversampling still gives decent results for my eyes at the moment.
And when I eventually move up hopefully next year, might look to get a 6700k platform when the new ones come out, and pick up the cast offs!!! which should help out more.
Any questions let me know
JP