I have never seen leather panniers fitted to either a G/S or an ST, I have tried using throwovers on my G/S but the strap between them was not long enough due to the offset of the exhaust. You might have to have some custom made to fit over the original pannier frames.
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