honestly i think that if you are using the wagon to shoot instead as a delivery-bus for your meganobz, you are wasting points. When i face a wagon with a cannon i am happy as hell, since they will scatter hard, have an insufficient range to shoot from turn 1 and still be unable to reliably damage vehicles or Monsters. also, will lose transport capability and maybe lose a turn to shoot instead of going full speed ahead.
My answer is, if you want to shoot so badly with a battlewagon, use two rokkit launchers. But if you just want to shoot, take some more lootas, flash gits or artillery.