Your numbers are way over exaggerated. Sure, in the worst part of a game, when you turn down your GPU settings, you could see up to 20% improvements, but most people do not lower their GPU settings low enough to be CPU bound the majority of the time, or even a little bit of the time.If you see Fallout 4 then a 6700k is already 30-40% faster than a 4790k. And there are other situations where a 6700k is 10-15% faster than a 4790k.
By 2017 a 6700k will be on an average 15-25% faster than a 4790k in modern 2016 end games. Most likely.
And a 6700k is already 40-60% faster than a 2500k. So the bare minimum to play modern games is 3770k or 4690k. And a 6700k does help quite a bit.
You need not upgrade a 4790k today. But in max 1-2 years you'll feel you have outdated hardware.
And just because 1 game, which is shown to be pretty poorly optimized, shows a big improvement in the worst case situations, does not mean all games will show the same improvements over time.