Using the 2x teleconverter makes it a lot easier to get a good shot of a hippo. This is not a high-speed action shot; she was moving at a slow walk.
This image was shot at ISO 5000, f/5.6 (wide open), 1/2000 on the Sony 70-200mm f/2.8 GM II with the 2x teleconverter. The focal length in the metadata is reported as 328mm. I wanted to print this image, so I cropped it slightly from 8640 pixels wide down to 8192, which adjusts the aspect ratio to fit the printable area on an A4 sheet. Apart from that slight crop you are seeing the entire frame, and I am satisfied with the image quality. If I zoom into the corners, I see a mild grain in the out-of-focus background; I like that effect.