How do I increase arty effectiveness against armor?

[Marcello]

First of all before messing with such established numbers I suggest you set up a pretty comprehensive test program. By which I mean setting up specialized scenarios where tank clusters are bombarded with artillery, the results noted and the tests repeated several times. That is the sort of stuff I did to to investigate some issues in the past, which uncovered some oddities.
Then you can tinker a bit with the HE penetration values and see what you get.