The best upgrade choice, CPU or GPU for these specs.

Currently I'm using 4700K @ Oc'ed at 4.3Ghz, running cool and rock solid, my graphics card is a MSI GTX 980 Gaming, slightly Oc'ed from the factory but nothing fancy.

The deal is that I'm considering upgrade my monitor to 1440p, while I haven't seen any game so far that I could not run at maxed out settings, with the notable exception of Witcher 3 in wich I play with hairworks ON but lowered grass drawing distance to high instead of ultra, I keep an almost constant 55-60 FPS under these conditions.

Now I know that going 1440p will hurt my current 'invincibility', so I've been thinking if It would be a good idea to go for a GTX 1080TI or non Ti and keep the 4770k, or should I actually upgrade mobo + CPU to the 8700K and hold on a little longer for a GPU upgrade, since a better CPU will also give me a performance boost... I'm really unsure, I've heard some speculations of the 2080 or 1180 being around the corner, but I can't confirm that, plus, I don't think i'll ever go anything above 1440p, what do you guys think?
 
Solution
4th Gen i7's vs 8th Gen i7
https://www.techspot.com/review/1526-intel-4th-gen-core-i7-vs-8th-gen/

Average 10 FPS on the minimum frame rate and 20 on the Max frame rate.

GeForce GTX 1080 vs 980
http://www.trustedreviews.com/news/gtx-1080-vs-980-2941822

Read the reviews, you will notice you see the most improvements from the GPU upgrade, yes the 1000 series replacements are suppose to come out in June starting with the 1180 Ti so it's really about what your willing to speed. Current gen cards you will probably see on sale by then and your 980 GTX will resell easy for around $200-$250.

delaro

Judicious
Ambassador
4th Gen i7's vs 8th Gen i7
https://www.techspot.com/review/1526-intel-4th-gen-core-i7-vs-8th-gen/

Average 10 FPS on the minimum frame rate and 20 on the Max frame rate.

GeForce GTX 1080 vs 980
http://www.trustedreviews.com/news/gtx-1080-vs-980-2941822

Read the reviews, you will notice you see the most improvements from the GPU upgrade, yes the 1000 series replacements are suppose to come out in June starting with the 1180 Ti so it's really about what your willing to speed. Current gen cards you will probably see on sale by then and your 980 GTX will resell easy for around $200-$250.
 
Solution
here is what you can do before making a decision,

open msi afterburner, run witcher 3 with highest details, is your gpu running at 90%-100% all the time? if yes, then i would say, get a better GPU.

if not, it means there are still juice left in 980, and CPU upgrade are likely to squeeze the last bit of performance out.