Welcome Kymco Owners!
Not sure that I understand completly your question (english is not my language) but curently, my photo backpack weights about 10kg. With the new setup (that plastic box alone weights 8+ kg and my extra gear another 2) the overall weight will be around 15-17kg. Considering that i want to put it along the seat (like as a pasager) I don't think that it will impact the handling. But maybe I miss something and that's why forums are great to point out