i'd do tile behind the cooker in any case. while i wouldnt mind cleaning water spatters off of the mirrors, cleaning grease splatters off of mirrors isnt appealing to me.

Glass of any kind behind a cooker needs to be tempered. You can get highly reflective metal that also gives a mirror effect

My sister did this in her kitchen (not behind the cooker tho). Thought it would make the kitchen seem larger - never again she said.

From your most appreciated replies, I understand using mirrors is not an advantage at all. I will opt for opaque tempered glass, I think . Thank you all :)