If not for this contrived 'test', I would not have brought the 100-300 on a birding outing at all. Outside of testing of this sort, doubt I'll use it with the 2x TC in the future. I can see using it with the 1.4x TC for outdoor field events, a 140-420mm f/4 zoom will be useful in that setting, and under non-pro field lighting the 100-500mm needs ISOs of 12800-25600, so the extra 2/3 – 1-2/3 stops of light will make a meaningful difference.