PUBG had a pretty decent performance improvement in the last patch... it's somewhat playable now.
The 8700K is the absolute best CPU for gaming right now, if you have the money, there's no point in not getting it...
The 8700 is locked, all motherboards currently avaiable for coffee are with Z chips which provide overclock ability, so i wouldn't suggest that.
The 8600K perform similarly to the 7700K from Kaby, which was already a great chip, if you don't have much to spend, it the wisest choice there and definitely will run PUBG and any other game easily...
It all comes down to how much you have to spend on it.
But aware that if you do decide on that upgrade, not only will you have to get a CPU+MOBO, but you'll have to get fresh new RAM sticks, as that platform requires DDR4 RAM and not your current DDR3.