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I have not used the 800, but I own the 600 mkI, which is heavier than and almost as long as the 800. I did use it with the sidekick for about 6 months. You will definitely need a low profile foot like the Wimberley AP-602 so that the lens sits over the ballhead. I found this setup to be useable, but since I have switched to RRS PG-02, I find the full gimbal to be preferable. I just feel like I have more control over movement (especially finer adjustments). If possible, see if you can borrow someone's full gimbal to compare with the sidekick. The sidekick can do fine in a crunch, but I won't be going back to it as my primary support.
Hope that helps.