If you are referring to the "6x" label at the periscope when you zoom to maxium power, that does not represente the amount of kilometers you can see, but instead the magnification of the power lense
In a real U-Boats bridge (Conning tower) you could see in perfect daylight (No fog, clean air...) conditions the horizon at 8000 metres. But you could see things that are tall (Masts, columns of smoke) much farther away, since they will pop over the horizon. While it is difficult to tell exactly how much, I can only say that the report I have read of a more distant sighting was around 25.000 metres, and it was smoke from a convoy that probably was several dozens of metres high in the air.
In all, the current 16000 metres visibility mod -which gives effectively around 18000 metres- is accurate for the 95% of the situations.
Hope that helps