increasing the pen on howitzers would go a long way to making them useful against vehicles.Howitzer pen is already enough to pen every tank. The problem is that they are real innacurate and are lucky to hit any tank even once. They are lucky to get even a single shell to land on a tank and a single hit of 160 or 200 damage isn't really significant. And tanks aren't exactly going to sit in the barrage area and wait to get hit again.
That's why they need more damage or crits to make every hit more significant, or stun to open them up to being hit again.