I carry 2 kayaks weighing roughly the same combined weight as your canoes. The only delta is I have Yakima crossbars, but the factory rail system. I have yet to see any reason not to trust the system, providing your load is distibuted evenly. Does this make it right...maybe not, but I do not think Toyota engineers, or any automotive designers, would advertise the max load capacity without a healthy buffer (liability concerns). Also, your roof rack might also experience varying loads with downforce when in travel, vice being parked.