From what I was told by a rep from Tamron UK (Intro2020?) the problem is hardware related and was due to the clever auto sensing system in the other lenses not being able to handle the magnification of the 600mm. I was under the impression that the firmwear update was to handle some camera specific AF issues, and had no effect on the VC system. Certainly in my use (glad it was a rental) for an airshow, for aircraft passing across the front of me out to about 60 degrees either side the useable image rate was 0! Outside of that where the camera movement is reduced significantly things started to improve. Head on and going away aircraft were fine.