Depends what you are going to do with the lenses. I have an RRS BH-55, and it's plenty strong enough to hold a 500mm lens, but actually using it is somewhat difficult. That much weight on top of the ballhead makes for an interesting experience when you loosen the main lock knob. Yes, it can be done, but it's not easy. Using a 300/2.8 is easier, but still not ideal.
If you want to do any sort of moving work -- birds, that sort of thing -- a Sidekick would work a lot better, and a full gimble mount is, of course, the ideal. But I think a Sidekick is useful just for general photography with those heavy lenses.