There aren't any 8 'core' Intel mainstream cpus until you get to the I7-9700k and up. Which is 2nd Gen lga1151, you motherboard is 1st Gen lga1151, so the 2 are not compatible. The best you can do is the 4c/8t i7-7700k, which honestly really isn't much (if any) better than the i7-6700k you have now.
Understand, the cpu sets the fps limit, it's on the gpu to stick that on screen or fail. So if the game allows for 400fps from the cpu, but your gpu can only put up 300fps and you have a 144Hz monitor, you'll see exactly no changes, gpu is still limited to 300fps, monitor can't refresh higher than the 144fps, even if the cpu was capable of 1000fps.
So with what games you play, at their settings, even a large OC can show 0 results for a lot of time, cash and effort.