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IS on that lens actually *works* on a tripod - unlike the many lenses for which tripod sensing simply automatically prevents IS from operating without you having to disable it.
However, as jcolwell pointed out, it's not for diagonal panning even in mode 2 and that is what often happens when tracking birds in flight.
Therefore you may benefit from mode 1 for shots of static subjects but most likely want no IS for general panning. A fast shutter speed goes a long way towards eliminating the need for IS but it often comes at the expense of higher ISO and hence both more noise and less dynamic range.
I've had mixed results with my wimberley head and 600 f/4 IS. Unless I lock it down and let it go I seem to cause lots of vibration by holding the lens and camera whether or not I am panning, and then I find that higher shutter speeds are the better solution. Despite this, IS offers advantages prior to shooting - a more stable viewfinder image for manual focus and a bit more confidence, and probably improved AF (because the image seen by the AF system is also being stabilized).