I think I remember reading somewhere that the Givi top box is rated to carry 10kgs on top of it's own weight (5kgs) but then that does have it's own bracket.
The truth is that the lower the weight the better, so put your heavy stuff in the side panniers and keep the top box for storing your helmet when you park!
I'm off for a three week ride tomorrow morning and have managed to keep the top box empty. Two up on the bike with full camping and cooking kit.
You just need to be disciplined about what you REALLY need.