I assume by your question that you are using factory produced flourescent fixture of the tube type. It's certainly not going to hurt anything by installing mylar behind the bulbs (lot of work though). I don't think it will make a big difference. Instead put a large sheet of aluminum foil over the tube to direct the light down. kind of cup it around and over the sides of the fixture.

i use aluminum foil....it's quite easy to thread it in between the tubes and the fixture, and then you have an extended reflector that can be shaped to focus the light where you want it....