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I've only used the mark 1 version of the Canon 2x TC, on my 70-200 2.8 IS and I was not impressed at all with the results.
Sharpness is noticably worse and focusing speed & accuracy is shot to pieces. Bear in mind that you won't be able to autofocus at all with the combination of 100-400, 20D and 2x TC. I've attached a couple of 100% crops (beloved of photo geeks, but useful here!) to illustrate the difference.
Also, by most accounts the 70-200 is the sharper lens, so if it's poor on that then it could well be worse on the 100-400.
Bear in mind that I'm talking about the older version of the convertor and that the new one is apparently better...
It had better be because I'll probably be buying it before next week! I'm facing the same problem as Mike Hoyer - short lens and shooting at Silverstone next week, so I need something longer.
For your own use, I'd just stick with the 100-400 - it ought to be plenty long enough.
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