The man made will work, but is slightly softer than the granite. I am taking a guess here but I would guess it is in the area of marble density.
BUT! You can get a Chinese pre polished stone counter top 8' long for about $125 if you look. It will be finished on the front and one side, not on the other side (if it bothers you buy the pads and hand polish the other end for about andother $150 of so). Makes an awsome bench top.
You were talking to a high end kitchen counter top manufacture. Not where you want to be for a workbench top.
Aaron