Been experimenting using a 2X Canon extender on a 100-400mm lens. The image below was manually focused at 800mm on a bean bag within a hide. Obviously, not a replacement for the dedicated 500-600mm lens but it's certainly an option for me for that bit of extra reach and certainly cheaper. Not persuaded the wife yet to let me buy a 500mm lens, but I'm working on it. I really liked the water colour in this image and its a bird I've been trying to photograph for a while now.
I think you did well to focus that combination. Details are a little soft, but certainly you were able to capture a useable image with this combo that the bare lens wouldn't have been able to provide.
The soft yellows in the water play well with some of the birds coloration.
For a moving subject, I think you did very well with that combination.