[mschuster91]

> how much safety margin does the tank have build in

From the article: 1 liter (747) to 2 liters (A380) per passenger. More than enough, even if a dozen people manage to catch the runs and eject more liquid.

> how would an almost full tank affect flight behavior

Not that much, pilots can (and have to, as part of weight redistribution caused by the fuel tanks emptying) "trim" the airplane - basically, applying an offset to the neutral position of control surfaces (see https://en.wikipedia.org/wiki/Trim_tab). And all the content of the toilet waste tank was already in the plane before (either in the bodies of the passengers, or in the freshwater tank), so it's only a minor distribution change.