OK, some news:
the ASUS KFNSE4-DRE does run on one 6 cores CPU, contrarily to what ASUS's
website states (I do have a rev 1.05G though but it's the standard version
IIRC, stated as not being supported).
So make sure you have the exact same board as mine at least.
Plus the CPU fan (logically) makes much more noise, which makes me want to
downgrade for desktop use. But at least I know I could give it a second life
as a powerful enough server.
Last but not least, it seems Coreboot landed me in some "jittery text mode",
going from SeaBIOS to GRUB was much faster, but the liveCD starts booting,
it's a bit slower, and it doesn't go all the way.
So, back to 4 cores.
It seems it's still about missing modules regarding GRUB, so I'll try again.
But the difference between "coreboot without microcode updates" and Libreboot
I've yet to make Libreboot really work (like booting the LiveCD, and not
having an abnormally long boot time).