You should upgrade your cpu to 6700k or 7700k if possible, or at least get a 7600k and push the freq to about 4.2+, upgrade to a better gpu don’t constitute too much.
I have two gpu rx580 and a 1080ti running a 6600k at 4.5 ghz, at 1080p,(ultra aa,pp,texture, very low everything) rx580 is doing 100-120 fps, 1080ti 120-140 fps., the 1080ti cost three time as much.
I can’t tell tbh without fps counter. So I would personally suggest upgrade the cpu if possible.