TCF is pretty hard, I don't know about TEF. I personally found it tougher than DELF B2. You can crack the writing part maybe by memorizing phrases and vocabulary but for listening, reading and speaking, you do need to have a good level already.If they study to crack the exam instead of focusing on learning the language they won't require anything more than a CLB5 (or even a CLB4). You can score way above your actual level as long you know the exam inside-out and strategize accordingly.
The biggest mistake people do when pursuing this French-for-immigration thing is actually learning the language. Learn the exam, not the language. Unless of course they want a non-IT Federal govt. job...