Hi Brian1965, thanks for replying. I believe the performance penalty is likely caused by the Spectre/Meltdown patches which were bundled with BIOS revisions after 03.91. I'm currently running 03.94 as there was a patch mentioned to prevent boot issues with Windows 10 which I am running on this system instead of Windows 7.
On the topic of the Z420 and cooling/overclocking: I have added the front intake fan which my unit did not come with, and also have both the front memory airflow shroud and a Z440 CPU heatsink on order as with the system OC'd it does tend to run a bit hotter than I'd like. I did apply some new Noctua NT-H2 paste today which dropped the temperature under load by almost 8-10*C so that's an improvement for the meantime -- however I'm wondering now if the airflow shroud and the larger heatsink will still fit together or if there won't be enough clearance? I didn't want to go with Z420's liquid cooler as I tend to stick with air cooling and the age/cost of available parts.