For more detail, according to that Digital Foundry review, the 8700K at 4.5Ghz compared to a 4790K at 4.5Ghz is:
38% faster in The Witcher 3
4% faster in The Division
7% faster in Assassin's Creed Unity
31% faster in Crysis 3
48% faster in Ashes of the Singularity
And if you look at the frametime graphs, it also gives you smoother frametimes, even in the games where the difference is minor.
Whether you think that difference is significant enough to warrant spending ~£700 on the 8700K+Z370 Mobo+DDR4, is up to you, but my annoyance was that comparing your own benches to reviewer ones isn't going to give you an accurate representation of the difference, hence it's always going to be a lot better to find a review that includes your CPU and go with that.