When it comes to teleconverters, everyone thinks 1.4x and 2x. I have used the 1.4x on my 300mm F2.8 on many shoots, and honestly I can barely notice any difference in image quality. 1.4x on a 300mm F2.8 gives you 420mm F4 lens. Autofocus is still plenty fast even with one stop less light. Put that same combo on a smaller sensor camera like the D500 and you have an angle of view of over 600mm and you are using the sweet center spot of the lens eliminating any corner image degradation. The 2x gives you a 600mm at F5.6. I have used this combo with good results, but some images do show loss of acuity and autofocus does not work as well (you have lost two stops of light).
But what about the 1.7x? I just returned from photographing bears in Alaska, and I used my D850 with 300mm F2.8 plus 1.7x converter for many of the images. This gave me a focal range of 510mm at F4.8, more than enough reach for many of the bears, and autofocus was snappy. After reviewing images I can report one thing; the images are very, very sharp (see images in this post). Honestly, if there is image quality loss on the corners I really don’t notice it, which may be due to most of my bears being in the center of the frame where things are the most sharp. I haven’t used my 1.7x on the 70-200mm F2.8, but I would expect very good performance. 1.7x gives just enough extra reach on my 300mm, and if I pair this combo with my D500 I am shooting with an angle of view over 700mm. I also seemed to overlook my 1.7x converter in the past, but based on the rigorous testing and field performance from last week, I will be using this tele-coverter a lot.