After reading the reviews here by some users who had issues with the older clamps, I contacted Thule support and they offered to send me a set of new metal hardware at no charge. The support rep. said that most of the problems with older design were probably caused by non-use of bow and stern tie downs, and strongly recommended doing this, even with the updated mounting plates. A few weeks later I received the plates, installed them, and everything seems to be solid but I think I'll still be using tie downs just to be on the safe side. Just noticed Yakima is now making racks similar to the Hull A Ports and also recommend securing the bow and stern. Guess they're playing "C-Y-A", too.
In any case, thanks to Thule for acknowledging the potential problem and making good on it. Oh yeah, the racks are great!