Hola Gerald

Muy can only modify an adjective or an adverb, so, in your example: Marco es muy trabajador , here, trabajador is not acting as a noun but an adjective. In your other example: Laura es muy cirujana, this use of muy with a profession sounds quite unusual to me, although as you said, you may hear it to emphasise how good a surgeon she is. The only examples that I can think of that make that emphasis and sound more natural and also colloquial are: 

¡Antonio es muy hombre! 

Here, "hombre" is definitely a noun, and this is something that you may hear meaning something like "Antonio is a very macho man". Equally you could also say: 

¡Antonia es muy mujer!

This would obviously mean the opposite to the example with Antonio.

But I can't really think of more examples using muy+ noun. I think this is limited to certain expressions like those I just mentioned. 

I hope it clarified it.

Saludos