While of course the rack's weight capacity rating is calculated partially based on the strength of the rack components, it's also based on the vehicle it's being mounted on. This includes the mounting method, such as when using the factory rack tracks/rails, but also the height & width of the vehicle. Putting that much weight so high in the air will have a significant effect on the vehicle's center of gravity, and therefore its handling. This was discussed at length in another tread a couple months ago. http://www.hobiecat.com/community/viewtopic.php?t=11367
Personally, I've been driving lifted 4x4s for years, and have carried many roof rack loads that have far exceeded the manufacturers specifications. While I've never had a problem as a result, I'd be the last to say there isn't a potential problem. Just make sure the racks are mounted securely, and that the boats are securely tied down. After that, just drive sensibly, keeping in mind that the handling is likely going to be less stable than usual.
- Jim L