I think most users have been using the small field for UW exclusively, available until now via a "workaround" described by coroander.
I just updated my firmware, and now the setting is easy to access: You need to make the grid with the larde AF areas visible on your monitor, then press "info" and choose your preferred AF field with the <up> and/or <down> buttons. That's it!
I am also very happy to see (at least theoretically, will have to make a field test) the new "low ISO" setting. When shooting macro, I sometimes like to use a wide open aperture, so I can isolate a subject (everything behind and in front of it is blurry).
In bright conditions with ISO 200 there often is so much light that the strobe fires at very low power, and as a result the red color does not show. With my old Nikonos RS and Velvia 50 slide film it was much easier, and ISO 100 with the E M5 is a step into this direction. One f/stop makes at least a little difference.
Picture quality? Well I'm not a "pixel peeper" and have been satisfied with the quality up to now. Maybe in certain situations (long exposure time?) there will be an advantage, may it be visible or measurable only...