The Wimberley Sidekick looks pretty cool for the swivel action. I would need a new monopod - the one I have is ancient, and cheap, with no option for a ballhead. I like the setup you have.
11.5 feet is not an issue for what I would shoot with it, and for the price it's good to hear of the quality. I wasn't doubtful since its a Canon but when comparing to the others it stands out being thousands cheaper than the next supertele.
When I shot the CCS races a few weekends ago, I disliked most images even at 1/800 shutter speed. I found them to not be that sharp. I was shooting with the XTi and an old Kenko 1.5x TC (which was discontinued in favor of the 1.4x TC). I cannot use it on my 6D, as it causes an error and the camera will not shoot even with manual focus enabled on the lens. Works fine on the XTi. I found that the shots at 1/3000 or faster were tack sharp no matter what, as I was shooting either handheld or with the camera sitting on a barrier.
To match the XTi + 1.5xTC at 300mm with the full frame 6D, which puts the effective focal length at 720, I would need the 400 and the 2x TC. However in reality, since I can crop off the 6D images, even just the 400 would suffice (compared to 450 on my XTi when taking out the crop factor). My preference would be to get the TC, because I believe that 800mm would yield much more detail vs. a cropped shot at 400mm.
This could be a nice "budget" 800mm super-telephoto setup for motorsports!