I have almost 30k miles on my 08's stock rack and OEM 39L trunk. The trunk bounces/wobbles at higher speeds (80mph or so) or over rough pavement, but everything has held together. I've run it empty to overstuffed, both in weight & volume.
I bought the OEM trunk thinking quality would be better than aftermarket, and that I would get a matching key. In retrospect, all I really got was and extra-cost piece of grey (neutron silver) plastic that says Kawasaki on it. Considering the price difference, I would just go w/ a Givi trunk were I to do it over. As much as the 39L wobbles on the stock rack, a more solid rack would probably not be a bad idea with a larger trunk.