So what exactly is causing the performance hits then? Shaders as you say? I don't believe that is CPU-limiting then?
My guess is shaders.. but it could be more than that. I don't really know the game architecture to give a professional opinion on it. It could be a lot of little things that weren't scaled back. This is the case with Watch Dogs and Lords of the Fallen. In order to hit that 30fps target, they took away a LOT of little things that was taking up the framerate.