actually, white walls would be better than mirrors. or if you can afford it, mylar. mirrors are made of glass that will absorb some of the light. flat white is a better reflector. looks like your walls are allready white.
Question: is there a heat issue in the room?? The reason I ask is I am looking at a setup for PERSONAL stash. I'm not looking for to grow 30 plants at once, but perhaps 2 or 3 tops. I have a fullsize room that is empty and calling for some plants. I was thinking of a switchable 250 or 400 watt hps/mh sungrow system. it's not the $$$ it's the not knowing what to buy. I will use soil to learn from and perhaps one day dable in hydro.
A typical mirror will absorb 15% of your lumens. It all depends if the glass is tempered or not. Mylar that place for 50 bucks or get more plants, or reflect the light better, put the plants right under that light. I wish I had that much space.