The results are shown below (you may want to click on the images to view a full-sized version). Not unexpected, the image quality is degraded by introducing the extender, whose optics are not as good as the lens. For the coarse details, the image quality is better with the enlarged, 400mm image, both in terms of sharpness and contrast. For fine-grained detail, however, the story is not so simple. The contrast is still better at 400mm but, a some point, the details are simply too fine to resolve at 400mm.
Not sure exactly how this applies to a bird image. I am thinking that for the Brown Booby images, where pixelation was noticable, I was better off using the extender. For the Osprey shots, I am not sure.
|With Kenko, 560mm f/8, original resolution|
|Without Kenko, 400mm f/8, image size increased in Photoshop by 1.4x to match pixel resolution|
|Without Kenko, 400mm f/5.6, image size increased in Photoshop by 1.4x to match pixel resolution|