I want it to hold as much as possible. The heavier it can hold, the better. If it's heavy, I heck it, do not mind. Most important is that it does not creep. I've got a 300/2.8 IS with me that I'm hoping one of those can support.
If this is the case, you might want to consider the wimberley Side Kick that adds on to any Arca type ball head. Much more stable and the ball head need not be rated that high. The other alternative is to have a dedicated Gimbal style setup.
Cheers,