From what I have read, it works with the Sigma 500mm f5.6. I don't think it would work with the Tamron 50-400, as I don't believe that lens was designed for a TC like the Sigma 500mm lens was.
One thing to be aware of is that Sigma lenses have no idea the Viltrox TC is there so Exif data will be incorect and it probably hurts AF more than a supported TC. But since Sony won't officially allow Sigma to support TCs, it's probably the closest we are going to get.
Does anyone have experience with this TC in the following use cases? I'm guessing probably not, because this is the equivalent of putting cheap tires on an expensive sports car, but here goes anyway!
1) Viltrox 2.0 TC with the 100GM for shooting bees and other insects in flight. Can you compare your experience to using Viltrox 2.0 versus Sony 2.0? Is it a noticeable degradation from the Sony 1.4TC if you are shooting at (actual) aperture of f/8 or f/9?
2) Viltrox 2.0 TC with the 100-400GM f/4.5 for landscapes. I am in the middle of a week trying this lens in the field as a potential upgrade to the 70-200GMii + 1.4TC for field sports and as a potential replacement with the 2.0TC to the 200-600G + 1.4TC for ~800mm compression telephoto landscapes. In this situation, all I care about is sharpness and contrast in (the increasingly rare) situations without atmospheric haze. I'm not concerned about AF performance or tracking capability with this combo, as I would be shooting static subjects
