From what I’ve read I don’t think temper is linked genetically to varroa resistance. Resistance to varroa is down to 3 unlinked recessive gene pairs - one for uncapping, one for throwing out the dead (more so than normal undertakers) and one clearing out affected larva. As the genes for these traits are recessive it’s very difficult to retain the genetics which is why bees developed for resistance don’t keep it - as soon as the next virgin queen mates with a drone with the more common dominant gene (non varroa resistance gene), the characteristic is lost in the next generation. Unfortunately many beekeepers think their bees are special and varroa resistant but if it was that easy we would have cracked it. I can see how aggressive bees might be even more angry though if they have a high varroa load! Having said that it’s a good thing to select for bees that have lower varroa loads providing their other traits are desirable too.
I also wonder if cross colonies could make more honey as the beekeeper is more cautious about going into the colony so they get disturbed less and get on with the job of collecting honey. I haven’t seen any scientific evidence that temper and foraging ability are linked genetically, probably governed by different genes and how frugal the bees are. The foraging site itself will have a big part to play in the amount of nectar collected and the size of the colony, as others have said
