Too many variables to hazard a guess, imo. The kind of light source (Single point source vs a T-5 tub or LED array) the distance between the light and the egg crate, the distance between the egg crate and the coral, angles of incidence, refaction, reflection, and may as well throw in the Redfield Ratio (as that might impact the clarity of the water.
The other issue is that with the egg crate in place it would be difficult to measure with a probe & light meter, but I think a par meter is going to be your best tool for investigating the impact.
If you are running LEDs, not at full power, you might be able to measure before and after and then adjust the output to compensate.
Enough of my messy guesses, lets see what the Board knows. (Bump)