A quick follow-up. The adapter plates I installed seem to be working very well and are strong. My bike was dropped on concrete (when parked) three times already, with the huge GIVI Maxia top case mounted, which completely protected the rear of the bike from sustaining any damage (can't say the same for the brake levers and the bar end and mirrors). I do have some scratches on the case, but the adapter plates are solid. Why dropping so much you ask? My daughter backed into my bike with the car coming our of the garage, then I did it a few weeks later, and I dropped the bike once when I thought the kickstand was down but it was not and I was in a hurry to get off it being late for work (in this last instance, the fact that I had loaded the top case with maybe 15lb of stuff helped me lose balance and made the bike top-heavier than usual).
As for "feel" with the large case mounted - I do feel cross winds and turbulence from traffic on the highway more with the case on vs. with the case off. Not a huge deal, but noticeable nevertheless...