H,
I've been thinking about why you have such great success with this particular lens and camera combo
My hunch is that the resolving "limit" of that lens matches the sensor is a particular mathematical way that you are hitting a theoretical maximum. A "sweet-spot" perhaps.
The newer sensors may well show up flaws or limitations in lenses that look good on a lower resolution sensor.
DT
PS. The above is probably just dribble and I really should just go to bed!