The latest compatible versions of XTU with Windows 10 (which do not cause immediate BSOD's) on the Z420 do not fully work as intended from what I can tell; for example the Additional Turbo Voltage slider is set to V (volts) instead of mV (millivolts) as it was in Windows 7 based on what I've seen from Brian's guides, limiting how much you're going to be able to push the system while maintaining stability.
In my approach, XTU is only used to unlock the maximum multiplier value so that ThrottleStop can do the rest of the work. Without XTU setting a higher turbo multiplier beyond the stock value in the TRL dialog in ThrottleStop has absolutely no effect, and even though "Set Multiplier" is checked and set to let's say 43, the processor will still max out at 39. I presume that XTU must be capable of writing to the MSR register for the highest allowed multiplier whereas ThrottleStop can not due to security limitations in Windows and the low level system driver which XTU installs (iocbios2.sys).
For documentations sake on the original post: I've noticed one change between the microcode versions from BIOS 03.94 and 03.95 which may explain some of the temperature differences -- the Tjmax value for the CPU is reported as 90*C on the older versions vs 95*C on the newer versions, I assume this is likely also having an effect on when the CPU is supposed to throttle due to TDP and when the fan is supposed to be ramping up (which was 80*C on older versions at least based on what I've seen here on the forums, and have noted a 5*C difference between HP Performance Advisor's CPU temperature reading and HWMonitor, Open Hardware Monitor, and Core Temp) as well across all versions tested.