Man, I have to disagree with you there. Everything I've seen of his performance so far implies an inner storm. There's a LOT going on beneath the surface. Though, I will give you the benefit of the doubt: all we've seen is snippets of footage. Context is everything. That said, nothing I've seen has indicated over-acting.
Also, though there are certainly not-so-subtle performances given by Affleck, I can't for the life of me remember when he last gave one, but it had to have been almost ten years ago. Gone Girl, The Town, and Argo are all GREAT performances, while Runner Runner is just blah (certainly not over-acting, though).
I agree with all of this and I do believe it's Ben's performance that makes him look so un-Affleck-like.