Prior to varroa, how many operations moved bees to Texas or California or other warmer southern states to overwinter?  How has that impacted and skewed data? 
Prior to varroa, how many colonies went to almonds?  How have the stresses, stimulations at the wrong time of year, high density hives spreading disease, etc. skewed the data?
Prior to varroa, what was the distribution of migratory pollination hives versus stationary honey producing hives? 
Prior to varroa, how many of the weaker colonies got wiped out by more noxious pesticides, leaving only the stronger hives for winter?
Prior to varroa, how many beekeepers were feeding pollen supplements?  Or corn syrup?
How have winter temperatures changes prior to varroa until now?
Prior to varroa, how many different queen genetic lines were available?  Now how many queen lines?  (17?)
How much fallow or scrub farmlands were there prior to varroa, compared to now?
How much livestock pasture prior to varroa has been turned into monoculture grain cropland now?

The list goes on and on.

<So my question is, what is the baseline to which we should compare today's
20-30% *reported* winter loss rate (note that this is a reported rate, not
an actual measured rate). 

I don't think we have a baseline to compare to.  There are simply too many variables at work that it is a fool's game trying to make a comparison.  How do you adjust for each variable?  It's like trying to compare apples and oranges.

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