I once asked the Ducati dealer the weight limit for the rack and he said it's the same as the one on Multistrada (?) which is 11lbs
When I got my rack I had it reinforced with "L" shaped plates before installing my GIVI box (can't remember the size or model of my GIVI)
Have a look at my photo gallery here. Some shots of the reinforcemnt and other needed stuff you should do i.e sealing your subframe ends.
I don;t think Ducati would entertain your claims if they find out you put a Givi on it
Btw, the GIVI box itself is about the max allowed weight for the rack. So I guess I have been lucky carrying my 600mm f4 L lens which weights about 12lbs. In total over 20lbs on the rack!
I have since removed the GIVI coz Canon says I should NOT be exposing the lens to such harsh conditions (vibrations, bumps etc)
I heeded their advise (canon's) only coz the lens cost about USD9,000!!
After seeing the pics of the cracked rack, i'm glad I did. Thanks guys