Well you can't use mirrors with sunlight, that's like taking a magnifying glass to an ant.
I'm trying one mirror with my CFL grow. I think low light like a CFL won't have enough energy to burn even in the hottest hot spot, but I guess we'll see. By the way I am using a first surface mirror, the kind used in rear projection TV's for the absolute minimum loss. (light doesn't have to pass through the glass twice, the reflective surface is on top of the glass.)
I may line the whole grow with them if I start to see good results.