For sure, and while I would be totally stoked to use even the orginal 600mm f/4 IS my ultimate lens is still the 800. For me I have made good use of even 1200mm on a 1.6 crop body with my 600. So it is not about whether or not which is better at 1200'ish range. The 800 goes to a whopping 1600mm! Also it would be closer to a bare 600 and 1.6 body like I was used to shooting for years, natively without extenders.

Not to say I never use a bare 600mm for birding. But I tend not to fuss too much about IQ and focus more so on the shot. And I have gotten some nice shots at extreme focal lengths.

Id have to look at the mark three version of the 600mm compared to the 800mm. But from what I remember there wasn't huge differences between the 600mm II and the 800mm when comparing them to equivalent focal lengths. In my mind it came down to personal preference and how you wanted to use the lens.

And a LOT can be said about the versatility of the 600mm f/4 IS II or III.