According to IGN it is based off AMD HD 6670... When the 360 was released I feel like it was much more powerful comparatively. I'm not a console gamer but I am still disappointed.
Edit: Rocksteady's Ninja Turtles, Xbox 720's chipset, I Am Alive Release Date - IGN Daily Fix Video - IGN
Edit: Rocksteady's Ninja Turtles, Xbox 720's chipset, I Am Alive Release Date - IGN Daily Fix Video - IGN