Currently playing at 2560x1440p and running gtx 1080ftw and i7-4790k.which one should I upgrade first for better performance?
I’d vote the CPU.
Any new GPU you buy will be severely bottle necked by that CPU so I would 100% say get a new CPU first also you will probably need a new MOBO as well.
CPU. You have a very good GPU and a fairly old CPU. Also WoW is a very CPU intensive game.
CPU 100%…

Ryzen 3600x +++ or Intel equivalent.
CPU 100%
Recommend an 8600k or 9600k.
Get the Ryzen 3700x
CPU that CPU was god-tier for its time but its getting to be a bottleneck for you, and 1080s will be perfectly fine for another year or 2 before you start having to drop settings/resolution
So AMD is announcing their new CPUs on October 8 so I would wait and look for that before spending any money.
For intel
10600k
10700k
10900k will need a beefy psu and cooling set up
I am assuming you want 1440p @ 144hz
Cpu hands down 1080 won’t bottleneck even 10900ks