Thank you for all the feedback guys. I guess I did not really explain myself to begin with, as far as the focus speed with a 2x tc is concerned I accept that it is never going to be blistery fast no matter which tc version you use and given this limitation capturing fast BIF is always going to be a bit more limiting, not to bothered about this (BTW I always used a focus limiter when using the 2x which obviously helped). My main use is for perched bird.
My main point was that I always found that it paid to stop down one to get the best IQ when using a 2x tc MkII (subjective I know but that was my feeling) and given the often gloomy weather in the UK winters, getting the shutter speed I wanted to hand hold the MkI IS lens at 600mm and f8 was more often than not very difficult (without using very high ISO's) - I was hoping that the new four stop IS would help a lot in elevating the problem, especially if the 2x MkIII tc was better when you were shooting wide open at f5.6.
I should also add that I am not interested in one of the bigger super teles due the weight/size - I am a weakling pensioner who gets all my shots by going walkabout along the estuary, ten years ago I would not have hesitated to get something like a 500/4 but age is against me now I am afraid.