Not a clue.

As you say, nobody has an MP-E equivalent. I'm a fan of limited DOF in general photography, so for me sensor size is more limiting, and would put me off 4/3 - APS-C has too much DOF for me most of the time. We all have our own reasons for choosing whichever system we have, but it's probably fair to say that for most uses, any system currently available is suitable for most people. If there was a compact 12mm lens for 4/3, then with the 25mm f2.8 and the 35mm f2, plus an E-420, you would have a wonderfully light and compact system capable of an awful lot - a modern OM.