I don't know, the U-bolt seems strong enough to me, considering I trust my own weight on 4mm and 5mm stem bolts. I snugged down the nuts with some grease on the threads and they haven't loosened yet. I do check them every so often, just in case. How much does your bike weigh? The heaviest one I have is under 30 pounds.
In any case, it seems like the mounting system for the square section load bars is clearly stronger than the one you are using. Spreading the load across two bolts at each attachment point has to be stronger. I didn't know there was another option -- it is not shown in the instructions for the carrier:
http://www.thuleracks.com/thule/inst...371,%20590.pdf
I have to ask -- is your rack made by Thule?