Turns out ArchBang installer is not loading microcode during install. Latest iso (2407) fixes issue.
Basically abinstall now checks vender_id and loads microcode into /boot. ArchBang iso runs both amd/intel but image is loaded via a loop device. So /boot does not contain microcode so does not get transferred to new install.
A fix for older installs will require you to reload amd/intel-ucode and update your given bootloader.
Grub for example:
sudo pacman -S intel-ucode # or amd-ucode for AMD
sudo grub-mkconfig -o /boot/grub/grub.cfg
More info can be found at Arch wiki Microcode
Thanks again to Barchie making me aware of this issue.
Mr Green