You were playing on a higher difficulty level than Core. The damage bonus also applies when you whack your own party members.
Taking this into account, the highest possible legitimate backstab using no mods except the Grandmastery Fix, and assuming you entered a fit of insanity and you decide to backstab your own party member on Insane Difficulty, would be...
Does some quick calculations.
...
Champion: Assassin->Fighter = 896 Dmg
Runner-Up: ToB NPC dualled to Thief = 856 Dmg
Consolation: Kensai->Thief = 816 Dmg
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