@NeroTheDawn Hi and welcome to Win-Raid forums.
The Asus Maximus Extreme 8 appears to have a very good 8-phase VRM - which is a good start if you are thinking of running a 6 or 8 core CPU.
Asus motherboards usually have protection against flashing a modified BIOS so it is highly recommended to purchase a hardware programmer (such as the ch341a) to flash the BIOS EEPROM and to be able to recover from a potential brick.
You should also make sure you have a complete backup of your BIOS chip that contains the motherboard serial number and ethernet MAC address for the Intel Gigabit ethernet adapter.
You will need to modify a BIOS that has Kaby Lake support, so you will need BIOS 2202 or later.
You can use CoffeeTime v0.99 to easily modify the BIOS as it has a nice GUI and there are fields to insert the serial number and MAC address.
You will need to make sure that the ME FW is version downgraded to 11.7.1xxx (or 11.6.xxxx) and swap the relevant CPU microcodes depending on which Coffee Lake CPU you buy. Make sure to leave the microcode for your Skylake (506E3) or Kaby Lake (906E9) CPU for testing.
Coffee Lake CPUID:
906EA = 6 core
906EB = 2 or 4 core
906EC = 8 core
906ED = 8 core
If you need help with CoffeeTime or have any questions just ask.
Once you have backed up your current BIOS and made the modified BIOS you are ready to flash it to the motherboard. Note that using software to do this is extremely difficult on Asus boards as the Flash Descriptor is often locked and difficult to bypas (read this) so a hardware programmer is highly recommended.
Once the modified BIOS has been flashed you need to test it with your Skylake/Kabylake CPU first and then insulate the motherboard pins before inserting any 6 or 8 core Coffee Lake CPU. There is a diagram of the pins you need to insulate in the CoffeeTime folders (CoffeeTime_0.99\data\info). It is easiest/best to insulate the pads on the back of the CPU with Kapton tape.
You will also need to connect the 2 pins that tell the motherboard that the CPU socket is occupied, for which a car rear window defogger repair kit can be used (although you can use graphite from a lead pencil for initial testing).