6600k is around 10% better. not much of an improvement in terms of gaming, but it'll be a few more fps.
If users are not upgrading from Skylake you more likely no need to upgrade, but if users are upgrading from something near end of life - Yes its a good offer to get the Intel i5 7600K instead.
Without overclocking, how about how much more performance will the 6600k provide compared to the 7500 (in terms of fps in game)?
You should get Intel i5 7600K instead if you plan to overclock. (Also you need buy a Good Cooler for tha as K version no longer come with a coolert).
(6600K will beat Intel i5 7500 and 7600 - (Not 7600K)