From my understanding, you need alot of vcore among other things to get the 980 and 990X to breach 4.2Ghz or so, so unless you have exceptional cooling, I wouldn`t even attempt to do so as there is a very high risk of frying something, those chips can draw and awful lot of heat/power and require alot of cooling and just the right settings for it to happen.
I have a freind whom has his 980X sitting at 4.3Ghz, I do not recall the voltages or anything, all I know is he had to have a custom water loop to cool it well enough to run it above 4.1 as it just needed alot of voltage to bridge the 4.1-4.3 range.
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