[chris_overseas]

Somewhat anecdotal evidence, but I was on safari with a guy who had a superzoom camera (can't remember which sorry but it went to 1200mm or so). He was mocking the size of my full frame DSLR + 800mm Canon prime, saying he had far more reach with his almost pocketable camera - which was of course true. I was curious to see what the difference was in practice so we both took photos of a waterbuck in the distance (him at both 800mm and 1200mm). Comparing them later on a laptop PC the difference was truly night and day, MUCH more significant than I was expecting. Every single hair was razor sharp in my photo while his just showed a mottled "mush" with very little detail at all due to poor optical resolution combined with overly strong noise reduction (the latter of which could at least be partly addressed by shooting RAW).

[semi-extrinsic]

I have a superzoom myself, and I'm not surprised at this. I guess you saw not just the difference in optical resolution, but particularly in hazy conditions, that the much larger front aperture on your 800mm prime means is "averaging out" blur from haze and air motion to a much greater extent, giving in total a much higher clarity.

In the middle of a hot summer day, the 800mm end on my superzoom is near-useless, but at or after sunset it's much better. Still very noticeable difference from your 800mm prime, I guess, but less so than at midday.