I concur.
Initial impression on the 863, So far on use, greatest downside of the 863 is the minimum focus distance of 5m and also the long focal length (makes it a very specialized lens) It is a very sharp and contrasty lense wide open, you can make up in the min focusing distance with cropping in if you have a high resolution camera. In comparison to the 400 f2.8 tc, it lacks the lens fn ring and doesn’t sport the new Silky Swift VCM, believe the Af will be slower compared to 400 f2.8, anyway usually af will also be faster with more light gathering as well. On feeling with Af for 863 is very fast but still need to use it more to get a more accurate feel as af speed defers in different lighting situation. With the 1.4x tc, sharpness and contrast is slightly reduced, stopping down to f10-f11 can visible increase the sharpness. I feel It is actually a very good portrait lens in terms of image quality and rendering, feasibility is another question. Greatest merits are the superb vr which allows very low shutter speed with high aperture (with tc or in low light) and the way it renders the image and then background. makes it extremely good for static or less moving subject or object shooting.
Also using the 500pf you won’t break a sweat, but the 863 with z9, definitely will sweat like a light workout. Planning to use it to lose some weight.