Hi Riekopo,
In our view, to model into the game the perfomance of Assault Gun, we decided to boost the soft attack and piercing stats. I explain to you. To destroy a bunker, you need a projectile with a large amount of explosive, to generate a sufficient shock wave, and a sufficiently large caliber (remember that the "powerfulness" of the shells goes with the square of the caliber). Obviously, the same goes for infantry.
So, more explosive shell=> boost soft attack, larger caliber=> boost piercing.
It also seemed consistent to us because the upgrades are a middle ground between artillery (enhances the soft attack) and anti-tank (enhances the hard attack and piercing) boosts.
What do you think? Do you have some suggestion for us?