I was very surprised at how much the canon 1.4x tc II sucks at af, it jumps back and forth 4-5 times before it focuses, compared to my tamron that focuses pretty much instantly
I tried this on my 20d with both the 100-400L and the 400 5.6L and had the same result, I figured the canon one would be better at af but definetly not. I tried it with and without the tape trick, actually without the tape trick the af did not work at all .
I have not looked to see the diff in IQ but to me its pretty much pointless if the AF sucks that bad you will not get many bird shots because by the time it focuses the bird will be gone lol
I will stick with my tamron.