I am surprised by this and wonder if it has not been damaged in an accident in the past, my GS, 160,000 miles and counting has done several overland trips heavily loaded on some pretty rough roads and the whole subframe is fine. I know of a few broken monolver G/S subframes but have never heard of it on a GS.
If it's in the Touratech catalogue then you don't need it
"You've got to think for yourselves! You're all individuals!"-Brian of Nazareth