no mirrors are not good for getting more light they actually change light energy into heat energy and create hot spots on ur plants same with alluminum foil so please never use this stuff if you want light reflectors buy some mylar its looks like foil but it works better.
I actually like the white painted walls better than using mylar. Mylar is really hard to get on the walls without any wrinkles. It is best to put it on the walls perfectly flat. But white painted walls work just fine.
Matte white is the best surface for light diffusion,superior in some ways to mylar,inthat it reflects light from many angles getting to all parts of plants. I wouldn't use any thing else .Matte "Superwhite "is really good.