Ultimate image quality or just the best you can realistically afford and are willing to carry? I would have thought ultimate image quality still comes from digital formats much larger than FF (not that I can afford to find out either!).

Do you have or know of any images where this can be demonstrated clearly? I understand the theory about aperture values on full frame allowing for finer control and shallower DoF but when it comes to differential focus am not sure it's all that clear cut if you take into account the "telephoto effect" smaller formats have.

I can accept FF is better for shallow DoF but I'd need to see proof to accept that you're better off with FF for differential focus (i.e., sharp subject against a blurry background) to accept it as a practical advantage. APS-C (and I assume 4/3rds) is far more capable of differential focus than a compact!