Who told you there was real footage in "Cannibal"? Avoid them.
As far as I know (at least it was the case) the real tape of the act has never come into the public domain. Not even leaked.
Big fan of "Cannibal" despite the issues I have with the music and the (bad choice to do it) English language acting.
Otherwise it's a pretty damn incredible work and superbly crafted.
'Nekromantik' - It's certainly extremely strong stuff and very explicit imho.
But there are at least 2 other films based on the case and they are not remotely as explicit.
One called "Grimm Love" and one (supposedly really awful) by Ulli Lommel called "Cannibal" or "Diary of a Cannibal".
My (rather extensive) thoughts....
http://www.beardyfreak.com/rvcannibal.php