What performance difference will I see if I overclock an i7 7700K to 5GHz vs stock performance? Is it worth to spend money on a water cooler to achieve the 5GHz oc without a thermal issue?
Unless you buy a pre-tested 7700K that is guaranteed to reach 5ghz there are no guarantees you can even get to 5ghz.
Even pretested you have to be willing to at least use the equal CPU cooling solution the tests were conducted with and also the voltages and BIOS suggestions they used as well. (Some just don't want to do that.)
Silicon Lottery sells pre-tested CPUs, http://www.siliconlottery.com