Comparing the performance of the i5-6600K and the potential upgrade to the i7-6700K.
Comparing the performance of the i5-6600K and the potential upgrade to the i7-6700K.
Then a i7 of any sort will be better. The extra threads just mean more flexibility and fewer issues compared to the old quad core/thread CPUs.
But only you can judge if the price is worth it, as that depends on whether you find a good deal on a used CPU or if you're spending a lot on a new one.
If money is not a problem go ahead and upgrade it. But asking me, spending money on an entire new cpu when you have one especially when you can overclock. As long as you have the cooler for it that is.