No the mirror WILL NOT reflect 'more light'.

It will reflect more focused and less diffussed light.

That is what the atmosphere we have does for us, otherwise, we would all be burned up fuckers smoldering and smelling of cooking flesh all the time.

The mirror reflection is too focused for growing plants. Reflecting light from a mirror is beamed right back at the same angle or degree of pattern, thus only a small focal point is reflected.

A mylar or white background will indeed reflect the same light over a much broader, more diffused area. Better for plants.

When filming video or photography, no one uses mirrors for light...too focused and too many shadows...if the surface isn;t hit with the main refelction beam, it gets NO light...again, like the astronauts in space....

The gloves they wear.....on the side facing the sun.....the temps will reach 280 F...while the same glove, on the other side in the shadows, will reach temps of -280 F.

Mylar and white act as broad spectrum diffusers, thus in reality giving off far more light over a much shorter distance.

HAPPY LIGHTING