At work we've got a gen 2 and gen 3 2950 and the gen 3 is quieter, but still loud, wouldn't like to have it at home, but the r710 is much much quieter by comparison, I don't mind having it on at home when testing things out, but it's still loudish, more a constant drone.
Without hacking the firmware I don't think there's a way to slow them down, I know if 1 fan fails the rest goto full speed to make up the airflow.