There seems to be a lot of posts across multiple forums related to one of two things... 500PF vs 400 f4.5 and 500PF + 1.4x vs 800 f6.3.
I think that the first one is the easiest analysis... If you need the extra 2/3 stop of speed, will occasionally use a converter, and have eliminated F-mount bodies from your bag, then the 400mm f4.5 is the way to go. On the other hand, if 400mm is too short and you will essentially "glue" the converter to your lens, then it might be worth a more careful analysis.
Point in fact, prior to the innovation of high quality and light telephoto optics, I shot a 300mm f2.8 (many flavors) as my primary wildlife lens. My affair with this lens spanned Canon IS L and Nikon AFS II optics. These lenses were great, but never long enough. My solution was the addition of the latest-designed 1.4x converters. Forum photogs and pros alike claimed the near lossless image that results from the addition of these converters. So, I followed suit, glued the converter on and shot. Throughout this time, I began to realize the subtle bokeh changes, the slower AF, and chromatic aberration creeps that were never present without the converter. I accepted the image reduction because I needed 400mm more than f2.8. When a 200-400VR became available, I bought the lens and shot it for a year against the 300mm f2.8 w/1.4x. A less than careful analysis of the images revealed a significant improvement in bokeh, CA, and AF speed in the zoom. Key point... even the best converters degrade images. I sold the 300 f/2.8 because I needed 400mm more than 300mm.... So, to those who are buying a 400mm f4.5, but really need/want 500mm +, I suggest you buy or keep your 500mm lens.
As for the 800mm vs 500mm + 1.4x kit... the above remains the same. If you need 800mm for the occasional shooting situation, a converter will save you money and you can get nice quality in good light. Lance's images are an excellent example of what is possible. On the other hand, if you shoot tiny birds more than other subjects, and find yourself needing 800mm on a regular basis,... get an 800mm lens.
For anyone who cares about my decision... Well, I find that 400mm does not cover my wildlife needs with an FX camera. I have the 100-400S, and I find it sharp throughout the range at maximum aperture. I find it to be at least as sharp as my former 200-400, and that was more than good enough for me. However, I used the 200-400 w/ the D500 DX body and it was an effective 200-600mm lens. Recognizing that my 100-400 is more of a landscape, macro, close wildlife optic, I know that the 400 f/4.5 would need a converter 100% of the time. If Nikon were to introduce a DX body w/ Z9 capabilities, my analysis would change.
As a result, I have permanently glued my 500PF on my Z9. Rather than add a converter, I treat the Z9 as a zoom and will crop down to a 700mm field of view. This combination is light, fast, and accurate. I have found that 500mm is the absolute shortest focal length I can use for 80% of my wildlife images... this realization now has me looking at an older 500mm f4 AFSII for when low light dominates the scenes.
Final point... were I into little birds, I'd have ordered an 800PF a long time ago.
regards,
bruce