As you point out perceived power is the result of a set of complex variables that would be impossible to reduce to a single formula. The amp output is one of these variables and as such has a value.
Amp's nominal power rating - any use?
I just paired a couple of Coincident Frankentein monos with my SF Guarneri Homage. The sound is great (fat, rich, dynamic, transparent) and sounds well with any type of music (opera, rock, electronic...). These are 8W monoblocks and sound like with more power or at least the same as my previous fabulous pass aleph 3 (30W class A SS). Of course it depends if tubes not tubes, class A or not, speaker sensitivity, impedance load, room dimensions etc, but what i see is that it's not a relevant criteria at all on its own. Maybe there should be some transformation formula to take into account some of these factors to get some Apparent or Perceived Power, but maybe it would be hard to take into account all factors. Any ideas, opinions, on this?