I love my 100-400 and wouldn't sell or trade it for nothing! But, It's way too slow in AF even without the converter for any repetitive BIF for sure! I've got lucky a few times but those were far and few between. But you know I love to shoot the "itty bitty stuff' and chasing a Kinglet around a tree the push pull makes up for any BIF flaws. I would like to see Canon upgrade this lens with a new generation IS. That might speed it up a bit. But then Heaven only knows what this lens would cost! I paid right at 1K for this lens new at Adorama and now it's going for a whole lot more just as it was configured back then.