Surely whether the FAC weight and speeds are inaccurate is sort of irrelevant. You have to honour and follow them regardless??
This question of inaccurate loading and non-reflective weights is a funny one. In my mind the airbus is the first aircraft I've flown that gives an indication of the size of the errors involved. I am very sure that any aircraft will have the same problem if standard weights are used for bags and/or pax. It's just that the airbus highlights it.
I find it hard to believe that loaders would be bribed to go 4 tonnes overweight (not saying you're lying gnadenburg!!). I have regularly seen the A321 at least 5 or more tonnes over the estimated weight and the A320 4 or more tonnes. No bribes required!!