At the risk of oversimplifying, following is a list of reported anecdotes that may be instructive to compare and perhaps gain some insights on what is possible, or not.  Assuming that people are being truthful in their reports, all of these scenarios have occurred or are occurring.  

Anecdotes type A - Importing genetic material form reputable sources with documented resistance to varroa, incorporating it through instrumental insemination, or isolated matings, beekeeper A rapidly gets to a high proportion of colonies that survive, show little or no mite damage, and when sampled have low densities of mites.  When beekeeper A brings material from different sources and runs it next to his improved bees, the new material shows contrasting responses to varroa, i.e. deaths, high numbers of mites, mite damage.

Anecdotes type B - Beekeeper B imports improved queens or selects queens from colonies with low mite densities from within an existing population.  Some level of mating control is attempted.  Colonies in the operation improve in survival, tend to have lower mites, but treatments are needed occasionally. 

Anecdotes type C - Beekeeper C buys queens advertised as improved.  Some daughter queens are reared, the rest of the operation is left as is.  No efforts are made at following the lineages of introduced material or at controlling the matings. No improvement in the mite situation is detectable.  Treatments are needed.

Anecdotes type D - Beekeeper D stops treating and propagates from the best survivors.   Mites are not monitored, colonies survive, are productive.  No other material is introduced for comparison.

Anecdotes type E - Same as D, but the effort ends in dead colonies.

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