Now you’ve made a choice with your M43 Panny, I’ll look forward, as always, to the results. I now, in re-reading my comments see that I completely misunderstood will.i.am’s add-on (and what you were saying). If all the iPhone is doing is providing a camera back, which I’d not thought, then it could work, but I do feel that all the clipping and unclipping in a hostile environment, where ruggedness is usually of an essence, is not likely to lengthen the life of either device. Then there is the change in size (4 -> 5), and interface that limits the life of the add-on.
So … I think your choice of the G3F is a good one. Consider my suggestion of lens cases however that you could clip to your belt, or bag, that would enable you to pop the camera into your pocket, or even better into a soft case that you pop into your pocket.