I am a complete fool. I didn't read your thread properly. Still, it is true that the shift to kaby lake is a lot of money (CPU + mobo + RAM)Warning: Spoiler! (Click to show)
Well, objectively there will be a performance improvement. But it's not enough to turn unplayable frame rates into playable frame rates, or playable frame rates to smooth frame rates. Especially when considering that you have a GTX 1070 and an overclocked i5 4670k. It won't make a noticeable difference, that would make you glad to have paid for a new CPU, motherboard, and DDR4 RAM.
Here's a great video comparing the i7s in games, from Sandy Bridge to Skylake (note that Kabylake makes no IPC improvement over Skylake, just clock speed):
From benchmarks, we can conclude that upgrading to the latest platform does not yield significantly higher frame rates. And remember, you are buying a new CPU, a new motherboard, and DDR4 RAM if you upgrade to Kaby Lake.
Even if the upcoming Ryzen chips from AMD are as good as Kaby Lake, there won't be a need to upgrade.
Of course this all depends on whether or not you personally think the improvement shown in the benchmarks is worth it.