These sorts of things always come down to lethal dose and the natural history of the organism.  Bees that already have significant parasites are usually weakened by other factors.  The relevance of which would have to be determined.  What is the physiological and ecological relevance of "highly concentrated Bt poison feed"?  It all comes down to the numbers.  I think the Kaatz study is shallow and not at all conclusive.
If Bt were effecting the bees it would be readily detectable in the hive since the brood would be little Bt factories.  Researchers would be looking at this first and are perhaps looking at it now.  Since there is nothing in the literature at this time I suspect no one has found it.  Also bees do not preferentially gather corn pollen. Also some researches have used Bt to control the greater and lesser wax moths (Galleria mellonella and Achroia grisella), which are parasites of bee colonies, and do not report an impact on the hives.  This reflects the taxa specific nature of Bacillus thuringiensis.

Does Bt play a role?  No one knows for sure, but it seems unlikely.