Roof capacities are rated very conservatively. A heavier roof load with a much higher center of gravity and the subsequent altered handling characteristics in an already tall vehicle may have exceeded the comfort level of Toyota's legal department. In addition to the Sequoia, I had a '95 Pathfinder and a '99 4Runner, with a similar set up that both held twice that weight, and I experienced zero roof-related problems over the two years I owned each of them, having loaded the tops approximately three times a month in that manner. All three are/were tough trucks.
Perhaps Thule took care of the bar droop; it's been a while since I've seen a saggy Thule bar. I can tell you I saw enough bent Thule racks on sedans several years ago to be dissuaded from purchasing the system over Yakima. If you compare the cross section of an older Thule bar (I haven't seen a newer one so I'm not sure if it applies) with a Yakima bar, you can clearly see the difference in bar material, thickness and design that would lead to this structural flexing.
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Mark
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