Kenko Pro 300 1.4x would work but without or very slow AF, so would the entry level ones. However, quality is bad with teleconverter, so could forget about it.
Yup, agreed. Better to get the Sigma 50-500 even if it costs more. There are very few instances in which a 2X converter would work with this lens even if you MF, even if you ignore the bad quality (quality of the lens is just decent and usable when wide open at f5.6).