The 2600 is down by about ~30% in average frame rates (107 fps to the 9600K's 150+ fps) in Far Cry at 1080, and down ~38% in F1 at 1080P....(overclocked, the disparities could be even larger)
That might make a difference if planning on 144 Hz refresh... (Admittedly, these large differences will only exist with the very best GPUs, and, be much less noticeable as one works down to midrange GPUs, and, likely non existent with my own GTX1060, etc)
Nothing wrong with the 2600 or 2600X....but , they are NOT capable of the same frame rates generated by even the i5-8400, and, the lead grows as you work your way up the processor stack. Is it significant with a 60 Hz monitor, or at 4K when the GPU is entirely the limiting factor, or with midrange GPUs fully saturated with any CPU at least as powerful as the R5-1600? No. But, you can review 1440P and 1080P results here, judge for yourself... Realize, once you upgrade GPUs later, the potential frame rate gains may be somewhat muted over those possible with faster processors...
View: https://www.youtube.com/watch?v=3rOVfeujof4&t=499s