For what it's worth, Im currently using Intel XTU 6.5.1.321 with a Xeon 1650 v2, Windows 10 v2004 (19041.450) and was able to successfully get the CPU to hit 4.3 GHz on all 6 cores by setting the values to 43x in XTU and observed the clock speeds in HWMonitor; also able to use ThrottleStop to allow it to step back down to lower clocks on idle to save on power/heat.
Rtibby, could you kindly post a screenshot of your XTU settings where you've raised the voltage in mV? XTU will not allow me to set a value that low, showing the first step of the Additional Turbo Voltage slider as 3.90625V.