Need some advice please. Will a Canon EF Mark III work with a Tamron 150-600 mm, f/5-6.3 Di VC USD G2, including auto focus or manual focus??
Expect manual focus, a 2x drops your 6.3 to f11 so even a higher end body I would expect it to manual. If you refer to a 1.4 at f9 it may be questionable. I should note I don't have this lens or teleconverter. Just reporting on the limitation of slow lenses with teleconverters.
i have the ef III extenders 1.4 and 2, i don'y have tameron but i have the sigma 150-600mm and they work fine on that, two things to look out for is the obvious electronic capability and the rear of the lens, if its got the rear element very close to the rear of the mount the extender lens will touch and not fit, in the case of the sigma which is ef mount its set deeper into the rear of the lens providing space for the extender lens to physically fit. not really much help i know as i don't have the lens but just my experience with the ef-s / ef type lenses, if it turnd out it doesnt fit you might want to also look at kenko or tameron extenders, for the lenses with close to rear elements that the mk III canons don't fit i use the kenko ones as they are ef-s type (marked dg on the ones i have)