Are you aware that the tubular steel Givi racks have broken in the past, as have the factory plastic racks.
I see no point in installing a non-Givi plate to it, then the Givi mounting plate. It's just additional weight.
Old news, IMO. Yes, I've seen the report of a broken GIVI rack here. I can not find another single instance of one breaking other than the oft referenced David Morrow rack. He was carrying a fuel cell on his. He says it was a 10 liter cell but underestimated what that weighs. Gasoline weighs ~ 6.15 lbs per US gallon, or 1.625 lbs per liter. So he was carrying 16.25 lbs of fuel. But the fuel cell weighs something too (at least 5 lbs is my guess). Anyway, this is the only occurrence I am aware of, and I even Googled for more.
There are many examples of broken rear sub-frames, but those seem to have happened mostly using racks that eliminate the hand rails not GIVIs.
The reasons that I'd consider replacing the Givi plastic plate with an aftermarket one would be to lower the trunk COG a little (the Givi plate has the trunk sit up about an inch) but more importantly, to allow a small amount of range in fore / aft direction for passenger backrest comfort.