FileVault security hole when used on SSDs
Your making a test user account with a short name different than the eventual user to be migrated is sound.

In practice, you will in time over write more and more of the data, but if you have the time to first establish a file vault key and have the drive completely encrypted before copying any sensitive data, you have a more secure system and can know that the data can be sanitized cryptographically as opposed to being over-written or actually erased.

You'll want to look for these lines in the diskutil cs list output to know it's ready for the start of data migration:

|       Conversion Status:       Complete
|       High Level Queries:      Fully Secure
|       |                        Passphrase Required