Quickest way to "destroy" the contents of a drive? Consider a drive protected with the latest iteration of Filevault.
If I wanted to make the contents of the disk unrecoverable, I could of course do a security wipe of the entire drive - which would undoubtedly take a while.
Is there a quicker way?
Can the key material be irretrievably damaged instead for example, in such a way the encrypted contents are useless? For example dd'iing over a specific offset of the drive?
The encrypted contents are already useless, unless you have the recovery key.
If someone ELSE has the recovery key, then change it.
From https://derflounder.wordpress.com/2015/02/02/managing-yosemites-filevault-2-with-fdesetup/

You can add or change recovery keys using fdesetup changerecovery. To
change to a new personal key, run the following command with root
privileges:
fdesetup changerecovery -personal
You’ll be prompted for the password of an existing FileVault 2-enabled
user or the existing personal recovery key. Once entered, a new
personal recovery key will be generated and displayed. The former
personal recovery key will no longer work.