I have a Hepco & Becker topcase and they recomend limiting maximum weight in the topcase to 5Kg for safety reasons.
The topcase is mounted behind the rear axle and is mounted relatively high and so, can cause stability problems.
I would prefer to see something which straps on to the pillion seat as this would be above the rear axle, not behind it.
I don't have a Norge, so, I can't really visualize the mounting arrangements.
Hope this helps
Cheers

Brian