This is from the user manual
"Maximum load, distributed over the roof bars, for a loading height not exceeding 40 cm: 80 kg.
This value is subject to change, please check the maximum load stated in the instructions provided with he roof bars. If the height exceeds 40 cm, adapt the speed of the vehicle to the profile of the road to avoid damaging the roof bars and the fixings on the vehicle.
Please refer to national legislation so as to meet the requirements on carrying objects that are longer than the vehicle."
It looks like the rof bars fit under the rails so shouldn't affect the finish.