This chip is a strange animal. It will crash IMMEDIATELY and hard reset the system if I go over 1.400V and put it under load. It also won't POST at 49x or 50x multi, but it will do 4.4ghz at 1.200V and 4.6ghz at 1.295V. 4.8 is its limit with around 1.375V so I pulled it back to 4.7 and added +0.025V for stability sake.
That is still pretty good. I'd be happy with 4.7 ~ 1.35V, would probably stay pretty reasonably cool too.
My last 3930k wouldn't do 4.7 under 1.425V or so, IIRC. This one I haven't played with, but the previous owner was using it at 4.8, or 4.9, so we'll see once I stop being lazy.