The Canon 400mm DO II made a step change to my images. Super sharp, and super quick to focus. Light enough to hand hold and carry long distances it is now my lens of choice, although I wish Canon would bring out the 600mm DO, they exhibited in 2015 at the EXPO. With a FF camera I often use the 1.4x III extender, it reduces image quality / sharpness slightly but is definately in the range of acceptable. I have never found the 2x III to prodcue acceptable results with any lens other than the 300mm F2.8 II, although I am soon to try it on the 600mm f4 II.
The three images below are all taken with the 400mm DO II, but using the 7D II, 80D and the 1DX II
Kestrel, Canon 7d II, 400 DO II, 1/1000s, f4, ISO 320
Long eared owl , Canon 80D, 400mm DO II, 1/1600s, f4, ISO 100
Leopard, 1DX II, 400mm DO II, 1/500s, f5.6, ISO 800