No amount of grinding with a carbide bit would ever be able to get the passages to the size of a factory housing. You would grind through the wall long before it was larger enough.
Not true, the exterior dimensions are virtually identical therefore you could potentially grind the passages to the factory size. BUT, having done this, I can tell you that you're better off buying a housing with large passages in the first place. Its extremely difficult to get into the angles of the passages and you'd end up spending more on a selection of bits that you will on the proper housing.
