In game submarine can be detected by visual sensors in range near 5 km and with flank speed from 7 km. So the question is: why we should set detection range of radar to 4.5 km? Look at the picture from your link (Range). The first radar maybe had bad results with detecting range. But in my modpack this type of radar is installed in the middle of 1942. I think that in the middle of 1942 the escort ships could detect submarine from 10 km when good weather conditions. To conclude I want to say that in game the main thing is balance.