In the last 3 years Egg Crate has gone from about $4 a sheet to $10 a sheet. Crazy!
Egg Crate will cause loss of light, it is blocking light, simple fact, however it may not be that noticeable to your eyes. Some corals will react, but you should be safe using it.
There are 2 sides to egg crate, it you look it is tapered, you want the thin side UP, This will focus the light more straight down. It will have a dramatic effect on "spread" if you are running halides. As the light spreads out further from going straight down more of the egg crate is blocking light. Having the narrow side up helps this some.