Actually the Wimberley II separates in two pieces that fit nicely in the Tamrac Expedition outside pocket. It is a good stable choice for shooting and tracking birds. A poor choice for shooting Pano but it can handle the big lenses and I have made a few 500mm Panos with it. Like you say the offset is not ideal. It would be a ridiculous choice for shooting dedicated Pano with a short lens, you can just get to much for the same money.
RRS seems to have the best options.