I crossed paths with a friend at a local park last week, and he kindly lent me his 2X teleconverter to try. That was before I even realized that I had left my 1.4TC at home! Awesome coincidence, and I was more than happy to get to try images at 1000MM. I have to say that although the AF does take a hit, especially for moving subjects, I was pretty impressed with the optics. Something to think about adding to the gear bag....
Canon 7DII + 500mm f/4 II + 2XTC, manual exposure, evaluative metering, 1/800s., f/8, ISO 800, natural light, handheld, darkened the pupil, a few of the worst offending specular highlights removed from the chest and reflection. Audio used.