I added a 2xIII yesterday with the intent of using it both on my 1Dx and 600II as well as my 70-200II and 1D4. AF speed wise, I was pleased with it on the 1Dx - AF is slower, but still more than capable of tracking birds. Initial IQ was also very good, so I am pleased with that avenue. This is with either lens.
However, on my 1D4, it is a different story. I have noticed my 1D4 doesn't drive my 600 bare or with 1.4x as fast as the 1Dx. To say it is slow wouldn't be fair, but it is noticeable. When I added the 2x, it crawled. AF speed was no better with the 2x on the 70-200, which was a little shocking to me. IQ wasn't a problem but focusing speed is a big one.
Is this typical of the 1D4? I know people have used the 2xs on their 70-200 for birds with great results, so I can't think that the AF speed I am seeing can be reasonable. Any thoughts?