I would look into some of the aftermarket racks because they are usually flat and will be more universal in the possible luggage options. The 800 and 1200 aluminum panniers are shaped differently, and I believe that they attach from the opposite direction.
If I could tag onto this, are there any soft panniers that would work with the BMW racks on a F800GS? I'd prefer to not buy new racks, if possible. I know that both are slightly (or not so slightly) odd-shaped, which could be a problem.
You could use the Mosko Moto bags with an adapter plate or probably the Wolfman panniers. There are a lot of soft pannier options. I have the BMW atacama bags which are made by Mosko, but my suggestion would be to get the Mosko backcountry v2. They have a better system regarding the auxiliary pockets in the front and rear.
BMW makes a very nice (slightly) expandable and waterproof piece of soft luggage that fits on the stock rear rack (designed for their hard top box). I can't recall its name, but I have it installed on my GS and I use it all the time when I go shopping.