from what i have read, they started attaching whistling devices to the bombs and removed the gear strut mounted sirens in order to reduce drag but still achieve the psychological effects they where after. There is even some video on youtube of an interview with a stuka pilot who said that he disliked the permanently mounted sirens because of the drag they caused and that even at cruising speed they produced a wail loud enough to be a distraction to the pilot.
still +1000 for the early stuka's inclusion, because of the performance differences and especially the different look of the early airframes.
