I had a low-end kappa case (well 2, actually, 26l and 35l) for almost 5 years on 2 different
bikes and have done around 50.000kms with it. It was rated at 3kg and up to 130km/h.
I've had it overloaded daily on both bikes, up to 15kg, and have done over 200km/h on a
few occasions with the 35l case. No issues.
The Shad cases and racks seem to have even better build quality (I tried sitting on my
DT300 shad rack, and I'm around 95kg) so I wouldn't worry about it.
You can always carry heavy loads on your floor board though (had up to 50kg there
on my agility 125 with no issues) if you feel uneasy about it being back there.