I spoke to a Triumph guy about the weight issue on the rack and he said (off the record!) that it was a stability issue that led the weight restriction (and also the fear of a law suit!) If you look at the manual Triumph give a weight limit of 5kg for a pillion mounted bag and I have yet to see a 5kg pillion passenger on a motorcycle!

The final comment the Triumph guy said was to make sure the rack bag was securely and tightly strapped to the sissybar, this will help spread the weight.