They mentioned above that it's only an a320 board, though they apparently have their RAM running at 3200 speed already. They didn't mention if it's a 2x8GB or 1x16GB kit though, and a single stick could be hurting performance if that were the case.
Personally, I would wait a little longer for Ryzens featuring the new Zen 3 architecture to launch, which will likely be within the next few months, to see what those have to offer.
If you did want to upgrade right away, an i7-8700K might be a bit of a waste unless you are buying one substantially discounted somewhere. The newer i5-10600K is pretty much a slightly better version of the same processor with more room for future upgrades on the platform, though pricing and availability isn't quite where it should be for an i5 right now. Also, if you are thinking of overclocking in the future (or running your RAM at more than DDR4-2666 speed), you would need a compatible Z-series motherboard for those Intel CPUs, along with a cooler that's capable of handling those processors with an overclock.
So, between the processor, motherboard and cooler, you might be looking at spending nearly $500 for the upgrade. You might be able to sell your existing parts to make a portion of that back, but it would still likely cost you more than a few-hundred dollars in all. You might get somewhat better frame rates, but since you are probably getting over 100fps already, it might not be too noticeable, particularly if you are using Free-Sync / G-Sync on your monitor. In fact, if you are using a form of adaptive sync like that, it may be better to limit your frame rate to slightly below 144Hz so that can remain active, otherwise you are likely to experience some screen tearing outside that refresh range. What monitor is it, and what frame rates are you typically seeing in the game?
I'm not so sure. According to that, their existing Ryzen 2600 with a GTX 1070 should be averaging around 217fps in Fortnite at 1080p low settings, which is what they are apparently using. >_>