So is Fable 5 so "good" that I barely noticed any difference when switching back to Opus 4.8, or is it because it's actually Fable 5 now?

I concur with all said by many on the distinct better quality of output and "feel" of Fable. Could indeed be placebo, of course.-

Two additional things are clear:

- This calls for even better ways, to objectively benchmark these systems

- Such benchmarking will get harder and harder to do in any objective way, as these systems approach actual intelligence.-