David Eyre has made some interesting and acute observations.  As someone who
probably handles as many bees in a decade as he does in a week I certainly do
not question his report of what he has seen but I do (with much hesitation)
wonder about his conclusions.  There are also gaps in the information he has
supplied and the missing information might be relevant.
Were the "exotics" he introduced of the same race as his usual strain?  Did
the (for want of a better word) hybrid bees produce more honey?
There is plenty of documentation to support the theory that improvements to
strains of bees can be made by introducing unrelated 'blood' from a different
pure bred line of the same race.  However, if the cross is made with a
different race , for example ligustica with carnica or mellifera, then
heterosis or 'hybrid vigor' will result.  Subsequent crossings may result in
mongrels of no particular merit except being able to survive and produce
honey and, more importantly for somebody selling queens, unpredictability as
to good or bad qualities.
One of the effects of hybrid vigor is often bad temper.  Another is better
honey gathering, if necessary by stealing from other colonies.  It will be no
surprise then, that the hybrid colonies will be the first ones to rob those
more afflicted by varroa.  They just happened to get there first.  The others
would have got round to it eventually.
With regard to the observation that temper often improves on requeening, this
has often been reported.  It shows that (in some ways contrary to what I have
just typed) in these cases the problem is not in the genes but in the rearing
or nutrition of the queen or the state of the colony.  I have often noticed
that the temper of an ordinarily docile colony will deteriorate prior to
swarming, even before cells are made.
 I suspect the timing of the split may be a very relevant factor.  If there
is a resulting shortage of nurse bees compared to the amount of larvae
(including queen larvae) to be fed then the queen may not be optimally fed
which may have knock on effects later with her production of queen substance.
 It will not, however, affect her genes and therefore her worker or queen
daughters.
Could it be that the hybrids are doing so well that there are more bees
between whom the limited supply of queen substance has to be shared?
I pose these thoughts expecting a vigorous response from those who know a lot
more than I do.
Chris Slade