I suppose it depends on the plastic? Hepco Becker now sell some very nice plastic Gobi panniers that are supposed to be as good as Aluminium.
I've just had a spill on my K75 and the plastic boxes exploded when they hit the ground.
I have Ally boxes on my GS and dropped it a few times - hardly a scratch.
Perhaps the boxes you have on your GS are tough - but I'm guessing they won't be?
Tesch Ally boxes are bullet proof! There are lots of threads about this on this website, so have a look around...
(I'm quackers about bikes)
(I\'m quackers about bikes)