I run Yakima LowRider towers and 58" bars with four bike mounts. Not sure what the whole setup weighs, but I'd say at least 40-50 pounds. Mountain bikes being 25-35 pounds each, I'm guessing it's about 160-180 pounds total.

The rails are not impressive. I'd use the adjustable Yak towers, the Lowrider Max, if I was doing it again today. Those have the different feet so you can get a more shape-specific grab on the rails. The rails themselves appear to be reinforced with steel, they have three relatively solid supports on each side, and it's really solidly bolted to the roof. I prefer the three mounts per side for strength.

My neighbor has a 2007 FEH with the same side rails, and they run a similar setup but with a cargo box for ski stuff. Aero lift at higher speeds is probably the biggest concern for those kind of loads. For me and my bikes, my concern is the side stress. But we tested the heck out of it, and it's working fine so far. I agree with your concern about the long term and side stresses. I'm sure you yanked on it, as we did, and saw some flex.