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I love the camera description of the feature "1/320 or less". What does that mean? From 1/320 to 1/8000 or from 1/320 down to 30 sec. I think they mean "slower".
And yes, there is absolutely no reason for electronic first curtain to impact dynamic range. So it shouldn't matter. There are very good reasons for an electronic second curtain to impact dynamic range as it does in the GM1.
Lastly, I'm guessing the 1/320 limit is because of a limit in syncing the electronic and mechanical curtains in their implementation. At slower speeds any mismatch results in vanishingly small errors, at faster speeds you might get exposure errors or noticeable gradients across the field. In a camera that was originally designed for EFC not an issue (e.g. NEX, Canon, GM1) but perhaps there were limitations in the EM1 when they added it as an afterthought.
Very nice that they included something like this in a FW update!