The 8700K has a 500 MHz higher base clock and turbo clock speed, averaging about a 6% increase in overall performance over the 8700. This stock overclock can be used on a H370 motherboard, but if you get a Z370 motherboard, this would also allow you to push it further if need be. At the normal turbo speeds, the 8700K runs at about 65C - 70C with a 212 Evo, and you could even squeeze 100MHz - 300Mhz more performance without causing too much issues with cooling, which could translate up to a 10% increase over the 8700.
Hope this helps.