I still don't see why a well-placed mirror would hurt anything. As far as I can tell, since the light is being reflected back to the plant, there is no loss, only gain. If you were transmitting the light through a piece of glass, I would avoid that because there will be some absorption. However, that is not the case with a supplemental mirror as you were going to let the light get absorbed by the floors, walls, dirt, and pots anyway. Mylar is clear like glass, so in the end you still have a metallic coating on a clear material to make it reflective, same as with a mirror. Glass is is a much better insulator than than mylar film, but unless you're lining the entire room with mirrors, it shouldn't matter much. That's my two cents.
