I'm inclined to believe that Buckfast F2 aggression is a myth. It wouldn't surprise me if for every beek who claims to have had an aggressive F2 Buckfast, you'll find another who has had one that's been perfectly fine. I suspect there's a strong element of confirmation bias involved, and that should a colony headed by an F2 Buckfast queen become aggressive, many will just say "Oh, well, that's how Buckfast F2s are. I'd best get a new queen." without actually stopping to consider any other possible causes. It would hardly be the first time a beekeeping myth had become "true" by repetition, after all.
I wonder if anyone has actually designed and carried out a reliable study to determine the reality? Or even if it's actually possible to do so, if the F1 queen is open-mated and therefore carrying indeterminate genes in the first place?
James