If it would be helpful, I'd be happy post a rawdigger analysis of an E-M5 RAW image of a gray card, or any other test I can easily perform. If so, please be very specific as to what you'd like to see.
Do you also have a Nikon or a Canon? I'd be interested to see if there is a difference in spot meter exposure settings. So if both cameras are set to ISO 200 and, say, f/8, and mounted with similar lenses, and you spot meter a neutral reference...is there a difference in the shutter speed?
I ask because I suspect that the EM-5 is overexposing by one stop, and that's why it's performance appears to be so good. When I compare the camera settings of sample images taken at Imaging Resource or DPReview, it always seems like the EM-5 is taking in one stop more light at the same ISO under the same lighting.
In nearly all cases, when DxOMark indicates a lower ISO than the rating, it's because the signal is boosted to the correct level. This isn't cheating because amplifying the signal adds noise. It's nothing more than how the manufacturer has decided to process the signal. However, in the case of Olympus's latest cameras...I think they're cheating. They're ISO levels measure so low because they're actually changing the exposure from what it's supposed to be. Basically, I think they're exposing ISO 200 shots as if they are ISO 100. As far as I'm aware, DxOMark doesn't test for the correct aperture/shutter settings.
Well, that's my suspicion but I don't have an EM-5 to prove it. And like I said...this is based on sample images (and the fact that DxOMark measure the ISO to be an entire stop off.) I could just be imagining it all.
Olympus used to indicate how it rates ISO (they used SOS like Pentax) but they don't list the method on the new cameras. Hmmm....