way back with the a mount camera sony A100 I had an idea I had two 2x converters put them together then put them on a 300mm lens well it worked but really not that well.
for me the 100-400 with 1.4 converter. the 200-600 although a great lens it is to long. I do most of my pictures off my bicycle and with the cotton carry and the lens hanging in front of me my knees hit the lens every time I pedal
right now, I am loading my camera on my bicycle, and I head out and look for wildlife deer, eagles, cranes, and hawks. when we get new snow, I switch to farm building and equipment