I recently got the chance to review the Sony FE 100-400mm f/4.5-5.6 GM OSS lens and Sony FE 2x Teleconverter for Photographylife. I had a great time with this amazing lens/teleconverter combo. Sony has done a great job with these two and I was extremely surprised to see how well the FE 2x teleconverter worked on the f/5.6 max aperture lens.
Here is an excerpt from my review with my conclusion:
"The Sony FE 2x Teleconverter is an important addition to Sony’s full-frame mirrorless camera lineup. It significantly extends the reach of the compatible Sony lenses for a relatively low financial cost. It is very small and lightweight and proves an excellent companion for long hikes and situations where the size and weight savings are essential. While its best to use it with fast aperture lenses such as Sony’s 70-200mm f/2.8 lens, the converter works quite nicely even with the company’s FE 100-400mm f/4.5-5.6 GM lens. There is a noticeable impact on overall sharpness and an increase in chromatic aberration, but the image quality remains quite decent. Overall, the Sony FE 2x Teleconverter is a capable performer, and I highly recommend adding it to your camera bag if you have one of the three compatible Sony lenses. "