I asked this question to a salesman and was told that metal knobs get too hot and that it wasn't a good idea.
I assumed he was correct but not sure.

Well that seems a little odd, since so many manufacturers designed their cooktops and ranges with metal knobs. . . .

Would love to be able to put Cornu style metal knobs on the Bluestar I recently ordered, doubt the hubs are the same size or the markings compatible.

If the store you are buying from has both your model and one with metal knobs on hand, you could pull one off and try it, though.