Hi Sven,
here in BC lower mainland with the lesser timeline of European/British occupation, and our much lower population,(coupled with the number of detectorists), sites with silver coins to find are getting very scarce. I have done multiple hunts at older parks and been skunked outright.
Stanley Park now closed to detecting shut down the oldest, and highest use site we had. Just takes one careless person, just like a forest fire.
I honestly don't recall digging a silver coin deeper than 7" since I started detecting in 1999, the majority are around 3-6" in depth with 4" a rough average.
I will give 5KHz a try in the next while with both the Impact and Anfibio, If I was specifically targeting deep silver in a park or school turf environment I would probably select the CTX3030, at least for the first pass. In a target sparse area I would try the 11" round Anfibio coil @ 5KHz second, trash then the 5x9.5".
Cheers,
Drew