In my humble opinion, if your not getting inadequate performance, try to get more life out your oc'd 2500k.
There really isn't much of a difference in percentage to justify upgrading from a sandy to a kaby:
https://www.youtube.com/watch?v=DxGge0tR4IM
I have my 2500k clocked at 5ghz and I recently upgraded my graphics to a gtx 1070 reference and i'm getting well over 100 FPS, utra settings in games like doom and gta V. Though there is an occasional dip in fps, for the most part I'm happy with a smooth, stable gaming experience.
If you simply have to upgrade now due to inadequacies in performance, if I were you i'd upgrade to an intel core i7 7700k build. This way you get a significant increase in performance (Justifying the upgrade to a higher percentage of performance increase), giving you the ability to run games on highest graphical settings for a good solid 5 years. In your future i7 build, you also likely won't have to deal with potential bottlenecking issues when you decide to further upgrade your graphics card.