Based on the elevation of the Western Heights (the rooftop observers being up about 120 meters), I think they would definitely be able to discern the mere presence of a ship at that distance under the right conditions. They are certainly not able to visually identify them as destroyers -- enemy or otherwise -- at that distance, but rather infer their identity because no friendly ships are reported to be near that area. The relevant passage describes what Bräuer sees, even through his binoculars, as a "tiny black cinder on the horizon." The more difficult part, of course, would be estimating the range. Without rangefinding equipment, the best the assault pioneers could do was make very rough guesses based on horizon distance, apparent size, and speed.
Ultimately, though, you are correct that this would be toward the uppermost bounds of visibility. I therefore present you with the Scharfschützenabzeichen: (&) for wear in your signature, if you wish. Keep it up!
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