This is a positive sign that the "thinking" is going in the right direction, and that GM wants to do business in a profitable way. This type of thinking (among other aspects) is what will get future buyers.
Its easy to say, go cut 400lbs off... actually doing it is going to be a challenge, and one thats not insurmountable.
For example, go pick up a rear view mirror from a 70's/80's car, and then pick up one from a 2005+ model... theres a huge weight increase... in 1 part thats not even structural.
GM just needs to trim weight on everything, not removal of mass but actual engineering to do the same job with less/lighter materials.