They should stick to the apsc sensor...the size is much better.
Of course a micro 4/3 with crop x2 is faster to focus...the lenses are all wide angles...a fov 50mm is really a 25mm lens...a fov 21 is a 10mm effective focal length!!!
I´d be happy with a contax tvs style merrill camera.
Short and slow zoom
I'm going to argue the idea that APS-C sensor size is inherently better than 4/3. It allows for a larger surface area to catch light, agreed. But what you get from having a smaller sensor is:
-Smaller, ligher lenses (lighter elements focus very quickly. As you note, they have a low focal length number, but they are not "wide angle", they have the same fields of view as any other system of lenses)
-Capability for better in-body IS (a lighter sensor means the IS can be more effective, see E-M1 handheld shots at 1 second)
-Deeper DoF (equivalent to one stop on an APS-C or two stops on a FF) means that I can shoot my m4/3 lens at F1.4, but have the DoF of F2 (APS-C) or F2.8 (FF) while still having the extra stop of light, meaning I can use one stop less ISO (or two stops less than FF) to get the same image, and this partially negates the High ISO noise performance that comes with the smaller m4/3 sensor.
m4/3 has trade offs, but I'd put the current m4/3 cameras as capable as thier similarly priced APS-C counterparts.
I don't think pairing with either Sony or fuji would make much sense. Both of those are proprietary mounts. Micro Four Thirds is a standard intended to be used by multiple companies. It would save Sigma from being at Sony or Fuji's whim.