Agreed. The first shot from the tank would be absorbed(while shattering plate), and by the time that the Tank would be ready for a second shot, it would be too late. The Armer on a tank would be useless.
Another point to remeamber is that not only does Atium allow you to see the future, it also increases your mental capacity. I can see this adding as a addition to the side of the Mistborn, They would be able to very quickly figure out how to out manover their opponent.