Another i5 2500k user here
I've kept the same CPU and upgraded the GPU 3 times, can't remember what I started with GPU wise but then went to a 7850 and currently have a 6GB 1060.
I run at 1440p resolution and haven't had an issue, I think you can get CPU bottle-necking at 1440p and above, though I've never noticeably come across it
though I think for 4k or such you'd definitely run into an issue with a lack of CPU power.