The paint falls off
The expandable panniers have silver paint that falls off and covers that ding at the slightest stone or bit of grit. I'd guess BMW didn't like all the returns and dissatisfied customers? IMHO just another sorry addition to BMW luggage like the touring panniers for the R (full of lock mechanism than jams and won't let you have the key out with the box unlocked, poor hinges, crazy shape).
Throwovers do catch fire on the F650 exhaust, I've seen the results! The paint round the filler cap on the GS cops it too.
I'd fit the touratech rack if you are going the soft luggage route. It also protects the exhaust. The touratech boxes are the ultimate way to go, but you need to be keeping them for years to get your monies worth.
Andy