I have the original one, and i own a first series i don't know if we have the same fork.
Anyway, acerbis tank is plastic made very light. Probably two kilos, the original is metallic, probably 4 kilos and more.
Acerbis carry 9 liters more, but gasoline is lighter than water, so probably 8 kilos.
At the end,a full acerbis is 6 kilos havier
Don't worry ride your bike and have fun as much as you can
Dominator bianca (compagna di una vita) '89
Dominator Camel (Ciro Limited) '89
Guzzi Nuovo Falcone 500 '74
Guzzi V35 cafe racer
Aprilia Tuareg Rally '90