I'll give it a try.
The 2666mhz is a speed that Intel has set that should always work,something the memory controller will handle,anything above that is seen as an overclock although spd speed for ddr4 ram lies at 2133mhz so anything above that is already seen as an overclock looking at the ram itself.
The difference between 2133mhz (spd speed) and above=overclock (XMP speed) is something else,the spd speed is something that will (should) work on all motherboards,the XMP speed is a higher
tested speed on which the ram should be able to work,but sometimes only does under certain conditions like an overclock on the cpu.
3200mhz will very likely work with that cpu without problems,but you might need an overclock on the cpu to get there which i don't think so right now but i need to mention that (overclock the cpu overclocks the memory controller as well).
In extend of your question you might as well wonder why to get a "K" cpu if you don't want to overclock.
Whether the extra speed of ram will help is something different,in some cases/tools/programs maybe even nothing,in some games it has been shown to give a few frames extra,maybe just keep the lowest frames higher.I'm not talking about benchsoftware here,which most likely will show a better performance depending on what latencies used with each set as well.
A set of 2666mhz with latencies of 14-14-14-30 might do pretty close to 3200mhz with 16-18-18-36,just as an example,depends on what benchmark as well probably.
This is how i understand things,maybe someone comes in and calls me an idiot.
In the end i think that you will be perfectly fine with 3200mhz and even higher seems possible with that cpu.
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