I think that Michael will answer to you better than I, by whom he has already had the 1DSIII in the hands.
But I believe that canon is not going to present a product, that in quality of image, is bad to a 1DS2 or one 5D, and probably approaches much the result of a P30,
Canon in the technical report spoke of an increase in the capture surface of pels, with respect to the 1DS2, evidently, as Michael says, will have to use the best lenses, so that any defect will be intensified by the hi-res.
Of all way, like all the others, we are to the expectation of which they give the new model to us, to make a personal test, of the yield each one is able to obtain, I believe that is only vay to obtain the peace of mind.