I should have mentionned that I mostly use the 400mm for Eventing and Dressage competitions. The Wimberly head is always on the move and not locked down. I am constantly panning and moving up or down pending on the rider and horse movements in Eventing. The IS is constantly trying to stabilize the image while I am constantly trying to keep a moving subject that jumps in a fraction of a second in my viewfinder while the autofocus and autotracking are doing there thing. That's why, I do not use the IS for Eventing competitions.
Secondly, I should also have mentionned that I use a DS MarkII. Not the ideal camera for the job. The autofocus and autotracking is not up to the job and when you take in consideration the obstacles, flags, trees, stectators; it explains in part my low count of keepers. When all the planets align the end result is worth the efforts.
I have 3 batteries for a day's shoot. Depending on the temperature with the IS on it's far from enough for the IS of the 400mm. The batteries are just not up to it. Please take in consideration that I shoot an average of 1500-2000 shots a day. I shot on occasions with a DS Mark III. Then I have no problem with the batteries of this camera while using the IS. Don't get me started on it's autofocus and the autotracking systems of the camera.
I use the 400mm on 3-4 occasions in a season. I should mention that my main lens is the EF 200mm f/2L IS USM. The IS of the latter is very quiet compared to the 400mm and does not burn up the batteries. Yes, the IS has far lens elements to stabilize and yes, I turn the IS off when the action starts except when I hand hold it.
IS is one of my most-prized lens features for shooting landscapes, candids and horse portraits, but I found it to not be helpful when the riders and horses are jumping and running at full galop. As stated, the IS is constantly trying to stabilize the image while I am constantly trying to keep an erratically moving subject in focus. Thus, you are quite right in stating that it is better to turn off the IS when the lens is fixed solidly on a tripod. Hand held for Candids and Dressage shoots are different, the IS does a beautiful job of it, it's saves the day!
Please forgive me, I should have been more specific.