From what I have read, there is about 4% loss before the light gets inside (skimming off laterally), 2% loss once inside the glass and another 4% loss trying to get out. They can almost eliminate the first 4% loss using anti reflective coating, eliminate the 2% loss by using low iron glass, and eliminate the other 4% loss using anti reflective coating on the exit side, creating a true 99% lens.
The standard borosilicate (high temp) glass lenses we get from China are not low iron and not AR coated, so they end up with about 10% loss total. When they call them 98% lenses, they are referring to the losses inside, meaning they are not using low iron glass.
Apparently the AR coating is not robust and not a DIY project
It would make sense to at least have it on the COB side and get that 4% back. All that said, I am still optimistic of the potential for lenses to help out our mid/hard-running COB guys to put the photons where they want them.