^ I think it's more of them moving Anne to Lead to avoid splitting votes with Chastain who is their best shot for a nomination. The performance buzz seem to be around her (and Matt) which is understandable since their characters are the heart and soul of the film. I do agree with you about Matt/Anne though as I think both the actress categories are weak this year.
Chastain has been receiving buzz for pretty much all her movies this year, and so far all of them have fallen flat. IMO, her best chance for a nom would be "The Most Violent Year", which would be bad news for Interstellar because double acting nominations are difficult unless it's a very weak year (a la 2008).