According to Canon's specifications, the RF 2x teleconverter is compatible with the RF 100-400mm f/5.6-8 IS USM lens (giving 200-800mm f/11-16). I use this combination on my EOS R. This restricts the AF area to a square (square with a side length of around 15 mm relative to the image sensor). The image quality is good, the relatively weak speed is evident from the data.