Unless that glass is tinted in some way it is unlikely to reduce the intensity of the light by more than about 10%. And, the 1/8" thickness is about as thin as it could be and still provide any protection for the LED. But, my DIY LED light has no splash shield at all, and it sits less than 2 inches above the water. It has been in use for about 2 years, with no problems.
I figured the glass wasn't all that necessary unless you wanted to keep the unit "waterproof". Even if it was a ten percent increase that would be a free upgrade that could be done by anyone who can remove four screws. I would love to see someone with a par meter pop out the glass and take some readings to see what the difference actually is.