I had a late 90's Chevy Suburban and used several sets of Thule 78" cross bars attached to the factory rails, as well as a rack set attached to the bare roof over the cab, and a few kayak stackers. With that setup we could haul quite a number of kayaks and even a raft frame. I think our record was 9 or 10, a mix of play, river, kid and creek boats, including a Dynamic Duo, and sometimes even a raft frame and rocket box.
The amount of weight we hauled up there always exceeded the manufacturer's weight limit. I have wondered if that limit refers to their cross bars (blades) and not the rails attached to the roof?
While that setup worked fine, it was a time consuming PITA to strap on so many kayaks and a drag on already poor fuel economy. Nowadays, anytime I'm hauling more than a few kayaks around I'll opt for the utility trailer which is so much quicker and easier than roof strapping, and less worry down the highway.