I don't think there is any easy answer to that. The roof is certainly strong enough to carry many times what the limit might be for stability considerations. The shape of the load and it's centerline orientation will make a difference, as will the overall weight distribution in the boat as a whole. If you've got an unbalanced boat to begin with, putting more weight on top is not good. If your boat is loaded evenly, and with the weight as low as you can reasonably get it, you can probably get away with more on the roof. Just guessing, I'd say if the total weight of your kayaks is under 100# you're probably going to feel OK with it up there.
I carry a 50# inflatable, and have a radar arch with the usual stuff on it, and have been comfortable with that. With what I carry, I'm probably more concerned about the increased windage than about being top heavy.
