These will be overwritten if grub is updated in a core update as those files are part of the grub rootfile set.
You would need to be careful with this. grub.cfg is a file that is created anew by grub-mkconfig every time a kernel update is done. I am not sure what might happen if you copied back a grub.cfg from an earlier version of IPFire into one with a new kernel.
Even if it does work then doing this will mean that any improvements, changes done to the grub system will never be available on your system as the restore will replace the grub.cfg with an older based version.
Your best bet would be to add your option into /etc/default/grub after any core update, if it is needed, and run
grub-mkconfig -o /boot/grub/grub.cfg
and reboot.
You could probably create a script that you would run after a core update has been done that checks if your option is still present or not and runs the grub-mkconfig command if needed.