AHB can stand the cold just fine. The problem with the migration of feral AHB colonies is that they swarm up to 18 times per year. With that much swarming, they never build up enough honey to last through winter. But put them in a beekeepers hive with 80 pounds....and they have no problems.
As for the AHB in south America, Yes, they are having good success with more gentle bees after many years. The problem comes when AHB are hybridized with the many strains we have in the states. To get to a point where they are in south America, you would need to let AHB completely take over as the only bee in the states. Something nobody at this time is willing to do.
Remember, as AHB takes over terrotory, you lose genetic material from the overall bee gene pool. The smaller that gene pool, the more likely you will encounter a disease or other problem that one strain may have problems with. But with various bee strains to always pick and choose from, the more likely you can fend off future problems.
The trick will be taking those traits favorable from the AHB, and incorporating them into breeding programs, yet not destroying all the other strains doing it.