You'll need photometric data from the manufacturers, or else you will need to undertake some calculations. There are means of verifying this information. What does the photometric data from the existing emergency lighting design scheme say? Are you borderline already, or do you have plenty of "leeway"?

Difficult to measure accurately because you will have shared light, the EM lighting must give adequate levels regardless of shared light, else you would be relying on the shared light in the first place would you not? As I said you really need the photometric data from the light fitting and check what this is on your existing design, then, you can get the photometric data with the diffuser and compare the two. It's not just the lux level but the spread of light that could be affected, hence the requirement for the photometric data giving the spread and intensity.