OK, it works ! ;D
Bootloader uploaded without any issue, and uploaded code without problem.
Will have to do further tests to determine if there is an impact on micros() and millis() or not.
Here is a speed comparison between :
Left: Adafruit Grand Central M4 120Mhz
Center: Arduino Mega 2560 R3 22.11Mhz
Right: Arduino Mega 2560 R3 stock 16Mhz
That is ridiculous !
BIG Thanks to all of you who helped me to successfully overclock my Mega
I've attached the files for those of you who want to OC their Mega to 22.11Mhz
my_boards.zip (219 KB)