I also use the Wimberley Sidekick with my EF 500/4L IS and love it. I purposely wanted a lightweight setup so its on an Acratech Ultimate Ballhead and Gitzo 1227 legs. I must have close to 10 years on that rig. I would say this is about as light as you can go - not sure it would work with a 600 f/4.
Dave
The Gitzo 1227 has been out of production for years and has been replaced many times since its being discontinued. I would also be somewhat concerned by the weight of the lens for that weight tripod at least unless you were with the tripod and either added or had the opportunity to add weight to the tripod.
