No, never even came close to any instability - solid as a rock. Can't say the same about my old ST1300's though, couldn't do over 120mph with a top box fitted - luckily never needed to

We'll have to wait until a few more new GS's hit the road, before we can confirm it's poor stability or not

It does say in all manuals that as soon as you fit a top box, only 130kph is recommended. I've tried sticking to 130, but they insist on putting up 50, 70 and 100 signs everywhere
