How about the 400/f2.8 + TC-20eIII? (And the 800 is NOT released yet.) I use the above combination routinely and so far nobody's asked me to back off to a better TC or lens. IMHO the TC-20eIII matches up better with the 400/f2.8 than any of the other lenses, although admittedly we haven't tried the 600/f4 seriously as we can't get it to AF with the 20eIII. Not up to racing work, anyway. I assume it'll work better than with my obsolete D3, though.
In #2, the driver has a clear face mask, and at 100% zoom it resolved eyelashes - at what Google Maps tells me is 290 feet distance. On a mere 12mp D3. I have no idea what the combination could do on a D800e, but I sure don't expect it to be worse than this.
BTW both of these probably could be sharper. They were shot at 1/200th or 1/250th to ensure some motion blur in the wheels. I'm pretty experienced shooting 800mm at low speed from a monopod, and these certainly don't look bad, but if one is really going for the sharpness gold, I'm pretty sure that a locked down tripod, mirror up and a remote will be better than either of these. Attachment#1 (jpg file) Attachment#2 (jpg file)
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!