I flew mine with the CG so far back that one time I was flying at Mendocino CA (Heeser St) and flew through a shear layer due to the sharp lip of the slope. My Zipper bunted violently, and began flipping foreward uncontrollably. It rotated all the way to the ground, landing in some brush. I was never able to get it to do that again, so I suppose I was right on the edge.

I've never seen anyone with the moveable CG, but I suppose its do able.