I own a 2X gen II and I have tried a 2X gen III. I think the gen III is marginally better and either will provide acceptable photos with a very sharp prime IF you don't compare the sharpness and contrast of the bare lens to the 2X images.
I have used 1.4X converters fairly often and now own an EF 200-400 f4 with the built in selectable 1.4X and that performs very well. But I have tried more than once using a 2X with my EF 200 f2, EF 300 and 400 f2.8, and EF 800 f5.6 primes and I have never been happy with the results from any of those combinations and the 200, 300, and 400 are razor sharp bare but even those suffer greatly with a 2X.
But the best thing to do is try to borrow one to use with your lens and see whether you are happy with the results. It isn't surprising that doubling the focal length of a lens via any sort of add on is going to negatively impact the optical quality, that is a tall order.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
