Found this according to the ASUS rep...
Quick Note regarding Voltage Scaling – Internal binning of both D1 and D2 parts we discovered consistent voltage scaling patterns.
1. For K series parts, the stock voltage supplied will allow for consistent overclocking generally up to a multiplier of 43x. There is potential for the multi to be raised to 44x depending on the load induced. This default voltage range be approx 1.240 to 1.260 under load.
2. Increased range between 44 to 47x multipliers will generally require a voltage range between 1.30 to 1.375V with an LLC recommended setting of high to ultra high.
3. Increasing the range between 48 to 50x multiplier will generally require a voltage range between 1.40 to 1.500 with a LLC recommended setting of ultra high.
4. Increased range between 50 to 52 (52 generally considered peak max multiplier except for rare 54x parts) will generally require a CPU voltage range between 1.515 to 1.535V with LLC at Ultra High and potential fine adjustments to the CPU skew range.
Overall a key item to note is the best voltage to oc scaling range potential for the turbo multiplier is 1.400 to 1.425 vcore. Using this voltage range with an LLC recommendation of ultra high will generally provide the best scaling potential with proper load temperatures*. We have generally found exceeding this voltage will not provide additional scaling or will increase load temperatures to a high level with synthetic load applications ( like Prime, Linx, OCCT ). Should you use more realistic loading testing (our recommendation is a combination of AIDA64 stress test, PC Mark Vantage) then temperatures under will be considerably under the max TDP rating.