There is a topic in the F80M3 section, where a BMW tech recently discovered that bootmod3 ecu flashes will be detected even after flashing back to stock + locking the ecu.
Worth a read through before you go the route of bench unlocking, bootmodding, and believing you can still claim warranty after something goes pear shape.
\\https://f80.bimmerpost.com/forums/showthread.php?t=1739110