CAFE standards were not created to reduce pollution. Remember that when debating this stuff folks.
Weight reduction is a great way to increase performance and efficiency if they do it right. Removing weight in the right places can lower the center of gravity and removing weight in general will help in nearly all aspects of performance. I doubt they'll be able to cut fleet weights by 15% by then, but it's not like it is the end of the world if they don't quite meet the stated goal.