On the mylar, I'd be inclined to put it on the walls, but not up around the light fixtures. Light from your T5's is already reflected downwards by the fixtures, isn't it? How about painting the floor white, too? That would bounce a lot of light back up for your plants to gather.
Old mirrors (garage sale or second-hand store) would be another option that would not present a fire hazard, and would be easy to keep clean. I have a ratty old full-length mirror that I move around outside to reflect some extra sunlight, during these short days of winter.