the safe voltage levels for the silicon are around 1.325V in high-current loads
and up to 1.47V in low-current loads
Really, take that advise for granted whenever your going to put in a manual OC. I've seen people degrade their 2x00 series chips to bits and even review websites sending out the wrong signal for putting 1.4 / 1.45v vcore. It will degrade your chip faster then you think.
Just as (my) 2700x; undervolting and good cooling is what gets the best out of XFR/PBO (4.1Ghz All core and 4.35Ghz single).