Hmm... Well I was looking it up and someone said
"Updating a primitive collider position is already costly, polygon colliders are really expensive by definitnion, i only use it for static objects, changing a polygon collider position is really expensive. And on top ot that changing a polygon collider actual polygon.? Its probably not a good idea, you could try but i dont thinl you will succeed. Most people use primitive colliders next to each other to fake a more complex geometry."
So, that's all I can figure out.