For example, take two subs each rated at 500w, one is spec'ed at 88db @ 1w 1m, the other is spec'ed at 91db @ 1w 1m, the second one would be expected to be louder.
too many factors to generalize that. typically yes but thats if EVERYTHING ELSE in the install is perfect (which doesnt happen often). with a good install a 250w rms sub can get louder than a 500w rms sub...all install....