I don't think the 100-400 is that awful with a 1.4X teleconverter. Here is a shot wide open at 400mm with a Kenko 1.4X teleconverter. EXIF will say f/8 and 560mm, due to the teleconverter....
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Here's another one, again at 400mm plus teleconverter and stopped down 1 stop....
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Of course, DOF is pretty slender and your focus had better be bang on at 896mm equivalent.