I am planning to buy a Canon EF 300mm f/2.8 L IS II soon, probably in the first half of next year. I plan for it to be my wildlife and birding lens, and with the 1.4x and 2x Mark III teleconverters from Canon, I'll also have 420mm and 600mm focal lengths. I'm a little bummed Canon does not make their own 1.7x TC, which would also give me a 510mm focal length at f/4.75 (not sure if that translates to f/4.5 or f/5 in actuality).
Are there any quality third-party 1.7x teleconverters that will work with the EF mount? I am interested in center to corner performance, however in the case of birds at 510mm if the corners are a little soft that would probably be ok.