Backstabs do massive damage in the normal game, only an uber or play dead watch can allow someone to survive a backstab. Another feature of the backstab is that it is often described as an automatic critical hit.
From TF2 Wiki.net:
Due to the nature of achieving a backstab, it is technically impossible to score a non critical backstab.
The large robots in Mann vs Machine can survive multiple rapid backstabs, and I'm wondering if there is a way to up the damage even greater. The spy can gain automatic criticals after a kill in Mann mode, and also a spy may use a canteen, medic, mark of death or jarata to gain crits or mini-crits.
However, if a backstab is already a critical hit then is there any damage bonus gained by crit-boosting a true backstab?
(Excluding face stabs, of course, which do gain from crit boosting)