F1? Where are you starting from? My F0 (if you like) are Isolated mated queens and their offspring (my F1's) are superb, the F2's are a lottery....and woe betide me missing a queen cell when they swarm.....
Ged's queens, as far as I'm aware, are open mated from Keld Branstrups Isolated mated Buckfast queens. So if you are getting 50;50 suspect you are breeding from what I would already call F1's....confusing isn't it when we don't use the same language
I hear of people successfully getting three generations or more from open mated Buckfast queens, My suspicion is these keepers live in areas where the local mongrels are placid so the aggressive genes are not present in the local drone populations. Be interesting to pursue that idea further.