Yup, you're right, I misread your post. Should have read it more carefully. I'm somewhat disappointed with the keeper rate of my copy when shooting hand-held, but I think it's just what it is. Your 20% keeper rate is indeed quite low, but it's hard to say what's really up with that.
Meh. I chalk it up to bad luck. I use the 100-400II + TC for general wildlife. Not a lot of BIF but I’m often unhappy about the F/8 max aperture vs available focus points. I also generally carry 2 bodies so another good wildlife zoom would suit me well. There is just too much evidence to say that the Sigma performs much better than I experienced. I asked the retailer to evaluate my return for a possible exchange. Hopefully it will work out. Like most people I’m losing faith in the Canon 200-600 rumour.