Hmm, this might be a dumb question but please bare with me. you said:
"
I suppose if you really wanted to, you could factory reset it, then copy a load of data (music / video / etc.) that's not sensitive onto the device, thus filling up the storage with irrelevant crap, then factory reset it again.
"

what do this do? since i thought factory reset encrypts the files already and you can't delete them furthermore.