Hans,
How can a Tenere owner throw the word 'ugly' at anything? And what does the fenderbag hold, a pack of three and a toothbrush?
I've had a variety of bikes with topboxes/bags and had no stability problems at the speeds I chose to ride at, which these days usually doesn't exceed three figures. Two examples:
two-up in Norway
Isle of Man, no passenger, 100mph over the mountain (legal there) no wobbles