Going off road with a top case is like riding in the back of the bus. Bumps are the most amplified at the back. Just mind the weight and look back now and then to see if it has become a dearly departed.
...and don't put your beer there!
BTW a 1200gs plastic topcase fits the twins optional oem mount. A cheaper option if you can find a used one. They are aluminum clad if that does it for you. Both our twins have one.