IMO there is nothing wrong with the factory cross bars and the box. When I was looking to see what set to buy years ago I called thule because they had a lower weight rating that yakima. When I asked why they said the problem was the uplift - canoes, etc. on the racks actually catch air and pull up. The factory racks could not handle this uplift. Since the boxes (at least on a car of this size) do not have the uplift that issue is out of the equation. Even if they had uplift my guess is the wieght in the box will compensate for it and/or the factory rack can take it.
Wit respect to the down force - both Thule and Yakima utilize the bars that run front to rear on the car as an integral part of the rack. They have a clamp type system that clamps to these bars and holds their respective cross bar; the only thing they do not utilize is the cross bars. So the real question here is whether the cross bars and the point they attach to the other (front to rear) bars is strong enough for the box.
I beleive they are strong enough for 2 reasons. First, I bet my eye teeth that GM/Cadilac has a box for sale in their accessories that looks identical to the thule/yakima boxes that they say can attach to the factory cross bars. Second, take a look at the load rating of the roof rack and cross bars in the manual for your vehicle. I am guessing that the load rating will allow you to carry either box on the factory bars - in other words the manufacturere says it is ok (I carry mine on the factory bars and have not had a problem!!).
Finally, for those that say you can carry more weight than called for in the factory manual by getting the aftermarket system - they are full of it. Both Thule and Yakima will limit the lbs you can carry in their box to the rating given by the vehicle manufacturer. Why - because it is not only the strength of the roof/rack that is imprtant in this analysis - it is also the actual weight that goes on the top of the car. By definition, when you add weight to the roof you are raising the center of gravity and thus the roll over potential - something that neither the manufacturer or the aftermarket guys are willing to take responsibility for!! The roof can take more weight than the manufacturer says (ie. the roof and rack will not bend, bucle or break) byt the dynamics of all of that weight up there is not a good idea.
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