Just yesterday somebody asked about this and got some info. Apparently even dirty glass isn't going to make much of a difference.
PAR Reduction do to 1/4" glass?
I bought the thin cheap acrylic stuff at HD and wow does it warp! I would not use that if lights are resting on top. If I flip it over every couple days it stays flat enough but if there was something on top it would warp enough to fall into the tank. In my defense it looked the same thickness and type as the acrylic top of the old tank, wrong.