Any of the Canon lenses will provide adequate resolution for up to a 24MB sensor. The problem, and especially apparent with a FF digital sensor, is that the lenses were designed for film. The photosites on the digital sensor are recessed so light that would have come through the lens and struck the film emulsion may be at too great an angle to register on the sensor. This is why there is so much vignetting with the 5D and lenses that are 24mm and wider.
Issues with lens distortion and chromatic aberration are more apparent with FF cameras which shows with the 5D where people forget how much better digital images with the crop sensors looked than with film. Some of this apparent gain in image quality was that the digital sensors only captured light from the center of the lenses where there was less distortion.
I would love to see images shot with the 1Ds and the 5D and the Mark III all using the Canon 24mm f1.4 and Sigma 20mm f1.8 lenses and compare edge to edge sharpness, (which is after all why someone would want a FF camera to shoot landscapes). I expect that the Mark III images would be noticeably sharper edge to edge and certainly it would not suffer from vignetting.